This is my first jam too! I'm also using Unity. While I don't have any experience to give advice from, what everyone has told me is not to worry too much about if you finish or not. Focus on learning something new and getting to know the community. To prepare, I've been practicing by following a ton of Unity tutorials, so hopefully I can adapt my skills to whatever theme is announced. 

Even though winning and even finishing isn't important, especially to us first-timers, some advice I've gotten and plan on following to make sure I submit is to get a working game as soon as possible and add polish and all that later. So then even if your game isn't done, you can still submit something that is technically playable for the jam!

Best of luck! I hope I can play your game when you submit it!