There are probably lots of things to consider. Only done two myself, but would say; dedicate time to brainstorming and planning, keep the game simple/small so you can finish it, prototype/build and test early, and go for a browser-based game to make it easy for people to play. Finally, I would recommend joining the discord server for the game jam. Usually, this is a good place to get help and encouragement. Hope that helps a bit!

Reposting what I said here:

For my own personal experience with last year's jam, my main suggestion is to quickly settle on what kind of game you want to make and keep it as simple as possible. Once you have something which runs, you can try adding more complexity to it. Having a basic game which works is much better than a complicated one which doesn't! The other (more important) thing I'd say is to treat it as a fun learning experience; doing last year's jam really helped me to grasp some very fundamental things which I didn't really understand at the time.

There was a livecast leading up to last year's jam which might be helpful (as well as the other videos in the playlist):

Or if you want a more succinct video, try this one: