So, I had never speedrun a game (excluding my one failed attempt). I really want to speedrun some game, but what game?

The standard reply in this case would be: What types of games do you like to play? What platforms do you want to play (PC, Web, consoles)? Do you want to play free game, or willing to buy new games, or want to play one of the games you already have?

From experience in previous similar posts, suggesting random games without info won't have any results.

It's not a question that needs to be analyzed to death. Pick a game, go fast. If that's not the right game for you, try a different one. You'll find something you like.

Sayonara Umihara Kawase or Inertia are the two games I personally run, feel free to check them out if it's in your fancy.

Im gonna elaborate on what Emerald a bit. I "pick up" new games almost monthly. Most of them get dropped by the end of the month because I just wasn't feeling it or it didn't click with me. There are even a handful of games I absolutely HATE as casual playthroughs but I'm still looking at as a potential speedrun (i call it spiterunning) because they have cool strats that i'm interested in. Might drop them, might stick. Who knows.

Just do it. Pick up a game, try out the run. If it doesn't click, try a different game.

pick a game you like playing and do runs of it, one of my earliest games I did runs of was Luigi's Mansion simply because I liked Luigi's Mansion