I just want to speed run sum cool games but I don't know what would be a good entry game.

So First Up choose a Game you enjoy or else you will hate that Game later on. There are also some things you like and not. Do you like long Speedruns? Then There are many Games for you. Do you like short Speedruns? Then There are also many things for you. My favorite Game, The Legend of Zelda Majora's Mask, is (for me) not fun to Speedrun. You Just have to find out which sort of Speedruns you like.