Well for game jam , all you need are : passion , time management and skillset.
advice :
1. first create whole prototype so at least you have something to display at end.
2. then start improving gameplay , art , sound and even create more levels.
3. Don't think too big in starting, it may lead to never completing game , so think small and then expand it with time.
4. always get help , search on internet , make proper plan of game before coding.