My biggest advice is to scope small. Try to get the gameplay loop done first, even if it takes you the entire jam to get it done. It's better to have something that's playable even if the game is only a minute long. Unless your game is a narrative game, I'd focus on the gameplay. Also, make sure to pick a type of game that's easy to code for your first jam. It's super cool to go with a game idea that will really set you apart from the competition, but for a game jam that (probably?) doesn't have any money involved, it's better to focus on something that you want to make but small in scope, even if there are other types like it out there.