make your first game to start out with to get to know the basics at the game jam is in 1 month you have time to prepare and more then when the jam starts idk? do a game you think you can complete in a week then 2 days to fix any bugs, upload to itch, and maybe add sound effect or audio and more, maybe plan after seeing the theme for the first 2-4 hr making stuff in you project and seeing what instrests and feels like a good game to you and for the jam. then after a clear thought just make the whole game around that. this is what I usually do (or atempt to do) any more questions feel free to ask (dc: _Abbas_Abbas_) and have a nice day.
(Edit: I didn't see your latest message sorry)