Hello! As for the game, i highly recommend to speedrun any game that you've played casually, as it'll help you to understand to game more easily with added speedrun route later on.
As for other game suggestion, i recommend checking out any Web / Itch.io games that exist on the page. Also, feel free to check out all the curated game list ( https://www.speedrun.com/games ) if need any other recommendation.
You can look at the top games at https://www.speedrun.com/games and see which one you want to start speedrunning.
you should start with some games you're really into already! pick a game you know you will enjoy playing a bunch of times, or play some flash games
Yeah, try running a game you already like a lot. Maybe watch some runs and then try it for yourself- and feel free to experiment with any ideas you have, especially if it's a small game or a category that not many people have tried!
Most of the games I run, or want to run sometime in the future, are games that I've played and enjoyed a lot in the past. With games I already like, I feel a lot more motivated to play more and to complete a run, or to try and contribute something.