Hello! It really depends on what games do you prefer to speedrun. For starters, i reckon you to speedrun any games that you casually enjoy just to get the hang of it. For other options, find the games that you'll think to be interested to speedrun on (you can find all the games in here: https://www.speedrun.com/games ) and try their shorter category (ex. Tree% on Minecraft Classic) as a test run.

I personally enjoy speedrunning any games that's either count as an "Indie Game" or anything with RPG elements to it. If you interested in that, check out my account as a reference to your future speedrunning game :)