Sayonara Umihara Kawase is my favorite game to speedrun, give it a go if it seems interesting to you. Simple and fun with satisfying movement, yet with a high skill ceiling at the top level: https://www.speedrun.com/sayoumi
For the past couple of weeks, i've been speedrunning Carto & Edna, and both of em are fun and easy puzzle game to speedrun. There's no major skips at the moment, so frame perfect movement and speed is the key to get a fast pace. https://www.speedrun.com/carto https://www.speedrun.com/edna_out_of_sight_out_of_control
Just watch a lot of speedruns of games you like/own/are familiar with and see if any of them look interesting to you. Taking random suggestions from strangers who don't know you never goes anywhere, especially when we have zero information on what systems you have access to and what kind of games you like.
I would suggest any games by Dema studios. They're all free, and relatively short but offer a lot of potential for speedrunning. I'm close to the wr in multiple of their games, so they're really good for beginners. From what I know The Pancakes Official Game Show and Bill's Hotel are the only ones on speedrun.com, but you should definitley check out both of them.