For this one specifically, I'd recommend going through Mark Brown's backlog of videos. The topic will be based off of one of them, so it'll give you a bit of a headstart when the jam begins. When it starts I highly recommend visiting the Discord as you'll be able to find constant support during the jam there. Don't be afraid to post screenshots, gifs, and prototypes to get feedback while you're working and to constantly reiterate on your core design. It's really easy to get sidetracked and focus on extra mechanics and unnecessary clutter if you don't remind yourself of the pitch. Stick to the pitch. Keep the pitch simple. I like using something that I would tell a friend about a game I like; "It's a rouge-like deck building game with perma-death." or "It's a puzzle game where you can change the rules by pushing text blocks around with your character." I hope this gets you started, and best of luck in the game jam!
To add on the pitch thing: the more of a mouthful the game is to describe in a single sentence like EATARI did, the more time it will take to make. Don't worry too hard about conveying intention or rules. Cheat if you have to by throwing everything in the manual, assuming that isn't part of the challenge/theme :P