You go to the board of the game you'd like to submit (You can use the search bar at the top of the site)

On that games page, there will be a button to 'submit run'. You will likely need a video of that speedrun, and to fill in some basic info
(Time, platform, comments, video link)

Then a mod of that game will need to verify it, at which point it will appear on the leaderboard. :)

Legs

excuse me, I can‘t submit a run. what's happening?

Please explain further.
If the run is not appearing in your Pending Actions, please see any of the threads made on the topic. It is a well known issue at this point.
If that's not it, we need more information.

even if I do not see my runs at the review, the moderators can see that?

Yes, mods can still see your runs even though they aren’t showing in your pending actions.