Games and interactive media form such an expansive medium that it can be hard to decide what to make at first.
Take a look at some of the game jam prompts, or google for a generator (e.g. this one). If you still struggle look at some submissions for inspiration, or even try to clone a mechanic that seems cool.
Try to limit yourself to a small set of core mechanics (even just one). Imposing limitations at the outset can make it easier to think up ideas.