Hi and welcome,

I'm pretty new to speedrunning too, although like you I watched for a while beforehand.

Personally, I would pick a game you are already comfortable with and can beat consistently on a casual run. Also, start small, so for example if you pick a game with lots of glitch exploits or manipulations, ignore them to start with and slowly introduce them as improve and get more confident in the game.

But most of all, just have fun.