rule 1. start small rule 2. don't use your first idea you might come across a better one rule 3. don't spread malware... just don't it sounds obvious but people do do it
I only participated in one game jam before (this will be my second time) but this is the advice I can give you:
Almost half of the time spent on the game will be beta-testing and bug-fixing. So make sure the game is "completely" finished with at least 2 days to spare.
Also VERY IMPORTANT to not forget to sleep and eat and do all of the stuff humans do to survive.
Oh and as kangsunsunwoo said, don't stop at one idea. I think it's good to try to define a bunch of ideas to see which one works best. And with "define" I mean try to _really_ imagine how it would look and play before deciding how good or bad it is.
Again, I'm not an expert..... But I hope this helps :D
