As @Sparrowkid123 said, a prototype is very important. You need to get the base game mechanics done so you can focus more on graphics, sounds and etc. Don't spend too much time on a single thing — if you can't get it right, move on to another thing, so you don't lock the game progress. Another thing that I personally think is good to know is: Fun > Realism. I use this concept a lot, because I'd prefer a game that's fun and unrealistic than a game that's realistic and boring. If you WANT to make a realistic game, I wouldn't recommend doing it in this game jam. It's very important to remember that the winner games will be in sites that host online games, like Crazy Games, so I don't think a realistic game would fit there, since the majority of its audience is made by kids.
These are some of my tips for this game jam, but take this with a bit of salt. You can do anything you want really, but just make sure it's at least a bit polished. Good luck on this game jam! Remember to have fun :)