I am really suspicious to talk, but i love shumps and they are quite good to start with. Them or platformers are easy game mechanics to make and learn as a first time dev. Try to pick a tutorial or a small course and follow it trough the end! Then choose a theme and create characters, new visuals or music based on your preference or imagination!
Idk if a game jam is a good place for your very first game, cause it can lead you to frustration deppending how you aproach it!