Keep the idea real simple for your first jam. Stick to one scene and just a few controls. Game jams are as much for practising as anything else.
A fun and short game jam entry is better than a long, boring one.
Don't leave submission until the last few minutes. Uploading your game can take a while, as can getting the settings right. For example, zipping your files correctly.
Also, make sure you actually submit your project via the button on the game jam overview page! It's easy to forget stuff like that when you're in a rush.