There's no real exact answer to this, but I'll try giving some pointers. 99% of people get started either playing a game they like or watching a bunch of speedruns they like and attempting one of those watched runs themselves (this is how I got stated). Generally I'd try to play games that you already played/enjoy. It also helps if it's a popular/well-developed game such as SM64 or Super Mario Odyssey that have beginner and advance routes mapped out in detail that are easy to follow. Another great place to look can be this thread.

As for finding good games for a specific console,
Click the 'games' tab (red circle in photo) and then in the top left, where it says 'all platforms' (blue circle in photo), click on that and select the platform you want to speedrun on. Then you will see games ONLY from the platform ranked in how popular they are (generally more popular games are more fun to speedrun)

First, check the game/category rules to see if they specify. Then, ask in the forums for the game you are asking about. If that doesn't get a response for 1-2 weeks, ask a mod. If they don't respond for around 1-2 months, see section 5 and go on from there.

Check the left side of that game's leaderboard (as seen in photo) to see if your console is supported. Most official releases of a game are supported by the boards. If it's still unclear, Ask in the forums for the game you are asking about.

First off, 99% of games aren't looking to add mods, but it doesn't hurt to ask. You should ask in the game's forums or message the mod either on site or through one of their contact methods if this interests you. Generally you won't be accepted unless you prove yourself to be dedicated to running the game you want to mod, so try running the game a bit first if you haven't already. The only other way you'd be accepted as a mod is if FAQ section 5 or 6 are happening, in which case you might be able to declare yourself as a mod for a corrupt or inactive board, but those are pretty rare cases.

First off, attempt to contact these mods if they are inactive (meaning your run hasn't been verified for 3+ weeks) through either the src messaging system or their linked social media. If there's no response for 1-2 weeks, go to the support hub and request to become a mod there and make sure you fill the requirements to become a game mod by clicking this text.

If you think they cheated their run, ask them about it, it's probably just some glitches or game-tech that you simply haven't heard about or don't understand yet. Alternatively, ask on the forums for that game about how it was done, or ask the moderators of the game about that moment in the run through a forum post, or a direct message if they don't respond for a week or so (sometimes cheated runs DO slip through the cracks). If the mod refuses to remove a clearly cheated run or fails to explain how their run wasn't cheated, submit a ticket on the support hub by clicking this text.

This varies based on the platform being played on, however, recording with a phone is ALMOST ALWAYS allowed (it should be always allowed as long as your phone clearly shows the whole run, but there's like 1 or 2 communities out of the thousands on site that don't allow phone runs). Check out the recording guide megathread
in order to figure out a basic setup, and check out the recommended capture cards megathread if you need external hardware or software to record your device or just want to see what people recommend to use for recording.
Also, there's some good options in the site resources

Generally, there's either some software or a capture card that can be used to record a game. If nothing else, recording with a phone or camera propped up between two textbooks or something is almost always allowed as stated before. If you want to find cheaper alternatives, or what most people use, it can help to make a forum post in the game you plan on running, since people probably already found the best way to record whatever game you want to run, or like mentioned earlier, the recommended capture cards megathread.

Generally, this is done with external software such as LiveSplit (what I personally recommend if it's available for you). If it ISN'T available for your platform or you're having issues setting it up, LiveSplit One is a browser based version that essentially accomplishes the same thing, which can be useful. Otherwise, recommended software can be found at the resource page.
You can also ask about a good timer to use in your game's forums to get a game-specific response, as there are some pieces of software made specifically to time certain games (including timers that work automatically without you needing to press and buttons).

Either ask someone in the community, watch another run, or you might have to find/make your own route!
Click here for a quick guide on how to make your own route (useful thread to read through).

Site rules: https://www.speedrun.com/rules
Moderation rules: https://www.speedrun.com/moderationrules
Game Request rules: https://www.speedrun.com/gamerequestrules

To change your Username, you can do so by heading to your settings here: https://www.speedrun.com/settings and choosing 'Username'. This can only be done once every 60 days. NOTE: Do not attempt to impersonate other users or staff members when changing your username, and do not use anything that can be considered distasteful. Everything is logged to site staff. Use common sense.

If you made a mistake when submitting a run, this can be corrected by first heading to this page: https://www.speedrun.com/pendingactions and selecting the run in question that you wish to amend. After selecting the run you will see a in the bottom right corner of your submission, click this and select 'Edit Run' then correct the mistake and re-submit the run. NOTE: If your run had already been reviewed by a moderator of the game, this will then require your run to be re-verified.

TAS and runs utilizing savestates are prohibited. TAS (Tool-assisted Speedruns) are documented here: http://tasvideos.org/ which is the preferred place for you to submit your TAS runs.
Segmented runs are allowed only if explicitly stated within a game's ruleset, or in categories marked as "Segmented". Segmented runs should not be passed off as Single Segment or Real-Time runs.

Official virtual console releases are always allowed, since they're official releases of the game.
As for unofficial virtual console inject or emulators, this is decided by a game's leaderboard. Check the rules to see if emulators are allowed or what the rules on emulators for your game are. If there is none, make a forum post in that game or message the mods of the game to ask about it.

Go to a game's leaderboard by using the search bar found next to your name and search for the game. Once of the page, find the category you completed a run for and hit 'submit run' from here, just fill out the info needed (if video is required, make sure it's publicly accessible) and hit submit and you will have submitted a run! These runs normally take around 1-3 weeks to verify depending on the game, mods, etc. and once it's verified, you get a notification here . If it takes longer than a month, look to section 5 for advice.

When submitting a run, submit something that won't be deleted over time and someone can access without having to sign up for something (so don't put something on google drive and share it only with mods). For example, if you are submitting a photo use something like Imgur, or Instagram, or Twitter. If you're submitting a video, there's a lot of options though. However, the most popular methods would be making a highlight on twitch (these don't get deleted), uploading a YouTube video and giving the timestamp where the run happened (hit the share button and change the 'start at' value), uploading a YouTube video of just the run, or doing the same thing you do on YouTube through something like Vimeo or DailyMotion. However, these aren't the only ways to submit proof, just make sure that they:
1. Are publicly visible (without the need to sign up for anything)
2. Show everything that's important
3. Follow the guidelines for proof in that game/category

In general, pausing is generally only allowed in RTA runs (real-time attack [meaning that you use a timer that isn't in the game, but just tracking your real time movement]), but not IGT runs (In-game time). However, this can differ based on a games leaderboard. You should check the rules to see if pausing is allowed prior to a run, as there are a solid amount of boards such as GoldenEye 64 where pausing in runs is allowed despite runs being counted in IGT, and some other games where pausing can alter game cycles. If it isn't specified, ask the game mods for the game or make a forum post in the game about it, but it's safe to assume it isn't allowed.

In general, all of these things are banned and considered cheating. However, there's some games where these things are allowed, and if they are it would be stated in the game's rules, so check there. In 99% of games though, this is considered cheating.

If a game brings up a question of 'why is this on the site?' it's generally because of 1 of 2 reasons.
1. Added by series mod
2. Added around 2-3 years ago or earlier.
3. It has a cult following of some kind or a large speedrunning community that you just didn't know about.
Only some time in around 2020 were games ever really rejected from this site. Before that, just about anything was accepted as long as you had a run of it. I'm assuming the policy change was made to stop the many dead boards from taking up site space, but idk.

As for why your game got rejected, from a users POV games are generally rejected for being
1. Too Short
Games under 5 minutes that aren't super popular also get rejected a lot.
If your game is short, try to find the longest possible category you can to showcase, as this shows your dedication to the game and often hints at potential for it to have its own small following.
2. Poorly Made + Unkown
If a game appears to be poorly made, it generally gets rejected unless it's a well known game. Stating awards a game has gotten or something like that can help your game not get rejected for this reason.
3. It's a highscore-based game with nothing to speedrun
Also, games like "Friday Night Funkin" get rejected because there's nothing to speedrun, and the game is completely highscore based.
4. It's not a game
Something like a 'discord ban speedrun" is not only dumb, but also not a game. Therefore, it doesn't appear on this site.

That being said, there's no concrete reason for why a game is rejected, as it is up to site staff to decide at the end of the day. It can help to read the rejection message to get an idea of why it was rejected, but sometimes the staff simply don't want it on the site.

Click here to read up on the guide I used to make this.

On the game page, hit "edit game" or "edit theme" to be able to change stuff from there.
Edit theme is used to change the theme of the game's page, which can edit trophies, background, etc.
It should be pretty self explanatory.
Edit game on the other hand has a lot more going on.
As you can see in the photo
the 'Info' section can be used to modify the links on the left side of the page, what platforms can be run, what regional versions are available, and a few more things which aren't on the screen shot, but are self-explanatory.
In 'Categories' you can set the rules for each category, including main rules that apply to all categories, and you can add ILs and categories with the associated buttons.
In the 'Moderators' section, you simply add or remove moderators or change people from verifiers -> mods -> supermods. It's very self-explanatory.
Under 'tags' you can define tags that your game can be searched under.
Under 'variables' you can set up subcategories and also variables that users have to input,
such as what version of the game they're running. If it's simply a normal variable and not a subcategory (which is defined by clicking the 3 dots), don't set a default variable, or variables that aren't the default won't show up by default when viewing the leaderboards.
For example, here's a normal variable, such as Bike or Kart and Game Version
Here's a subcategory

As for what categories/levels/rules to have, that's up to the moderators (such as yourself) to decide. However, it's always a good idea to keep community feedback/desires in mind.

That's normally up the mods discretion. Either make a forum post in the game's forums, or DM a mod asking if they would be OK with adding the category. Most of the time, if you've already done a run of the category, and it's possible to 'speedrun' it (so no high-score stuff or anything like that), then it will normally be accepted as a category and the mods should be OK with adding it. If they deny it, they'll generally give you a reason. If you ARE a mod, it's your choice what categories get added, but try to keep the community happy.

Just submit your new time with the proper date. The mods will verify both runs when they get to it.

Just type them, typing stuff like 'p0g' aka will instantly turn it into the associated emote.
To check which ones are available check https://www.speedrun.com/the_site/thread/4dilr
or
https://www.frankerfacez.com/channel/speedruncom_ffz

This is the official FAQ section added before ELO took over this website. Click here to check it out. Most of this FAQ has been integrated into this FAQ thread.