Build it.

Normally I'd say, "First check whether similar apps are already out there."

But who cares if you're replicating something that has already been marketed? You're 18! Nearly everything an undergrad will do at a university is redundant. We learn first of all by reproducing what's been done. So even if your app is "just another [fill in the blank]", you'll know how to run with your next newer idea. Anything you build looks good on a resume.