There's no substitute for just jumping straight in. Think about what games you would like to speedrun, maybe make a shortlist of games you enjoy playing and would enjoy speedrunning, then decide on a game and just give it a go. After your first run, seek out that game's community and ask for critique on your run, and where your biggest areas for improvement are. After that, it's a continual and gradual climb towards getting a better time. If there IS no community for your chosen game, that's fine too, as it's a rewarding feeling to do your own routing and to be able to successfully critically analyse your own runs and judge how you can improve.

https://www.speedrun.com/iwbtoretueeeee is short and even has an autosplitter. very easy to learn.