1) Relax, don't stress over the Jam. If you're stuck, you're stuck, step away for a minute/hour/day, do something else and see how you feel when you come back.
2) think small. maybe even a one screen game. Especially if you're fairly new to game dev.
3) Your first idea might be your best idea. if you still cant think of a game, start working on your first idea.
4) if you cant think of anything for the topic... google it. "define <topic>", "what is <topic>", "how do I <topic>", "what do I <topic>". "What do I rewind." Cassette tapes need to be rewound. so my main mechanic is rewinding cassettes.
5) Most importantly of all, have fun! You do this as a hobby, for fun. If you're not having fun, then don't, nothing is going to change if you dont bother doing it. I quit my first game Jam (this is my second in all honesty), I overscoped stupidly. who has time to make a complete RPG from scratch in a week? :p It got to the point that I started stressing about the product and it was only half way through. I dropped out, I wasnt having fun anymore. But that being said, this jam I didnt overscope. I had alot of it done in 24 hours because I was having fun. Then many hours after I wasnt in the mood to continue so I didnt. I worked on it maybe 2 hours all today. But the point is to have fun. If you're not having fun then dont do it. :)