Hey there friend! I know it's hard to make something so fast here are some suggestions:
1) Start the jam either with a core idea or a team of like-minded people. Check if you like the same kinds of games and if you have the needed skills to complete a game.
2) Once you decide on the core idea try developing further. Picking one or more games as a reference would be nice but remember people don't like pure clones, think of something that makes your game different. And it can't be "the awesome devs" or "love for the genre" or something like that. It needs to be specific like the controls are reversed, in our story you are the villain, you can't directly control your characters, etc.
3) Now that you have a better idea of what you wanna do you need to split your project into small tasks. What would you need to make a game? Well the main loop is jumping around, so you need a task for coding a jumping characters, you also need levels so you need a task for designing levels, you need a gameplay music so make a task for that, etc. You can use Trello for instance.
4) Once you split all the tasks you would need to accomplish to get the game done you now need 3 things.
4.1) You need to assign priorities (for example high, mid, and low) to each task based on how important that task considering the importance of the feature. You could have a task for creating a title screen but you don't need a title screen so that could be a mid-priority. If you are making a platform you need jumping mechanics so that is a high priority. Always work on tasks based on their priorities, higher priority stuff should be done first and should take most of your time and concentration.
4.2) You need to assign someone responsible for each task. People need to know who needs to do what so you can communicate with the team.
4.3) You should have deadlines based on the task complexity and the project length. This will help you deliver things on time and check if the scope of the project fits the deadline you have.