I did my first game jam last year, and have been enjoying them ever since. To list my tips I guess;
1- Don't over focus on polish - Come up with a plan you think you can get working and just get it playable, if you can easily polish it off with extra time at the end that’s great, but you don't want to spend so much time perfecting a mechanic you don't have time to make the actual levels to show it off.
2- Level before mechanic - This is my personal bias, but if you can't think of what level you could make with your mechanic, what kind of obstacle or activities you can do with it, near the start you may want to try something.
My first jam I had a character who could shrink, or go through certain objects. To come up with my puzzle I thought of the simplest obstacle that would apply to my game, a door, and ways I could show off my mechanics to get past said door.
3- Making a whole package - When making games we tend to get stuck focusing on one element of production at a time, game jams are a great chance to widen our view and focus on the entire development cycle in a quick burst. Come up with something small and simple, then plan out your time so with the aim of having a good finished package at the end (no matter how rough).
4- Have fun and don't overstress it - Have fun, don't worry if you coding is a bit shoddy or arts a bit of a mess, try to come up with something you think you can do, and see how it pans out, you will find you'll be much better at the end of this than the start.