It’s always best to work on something you’ll actually use. Maybe think about an app you already use but want to improve or simplify (a to-do list without the frills for instance).

As far as learning, I think this Stanford course is the best way (though not for those who aren’t absolute beginners to programming). I notice it hasn’t been updated lately but it should be current enough https://www.youtube.com/watch?v=TZL5AmwuwlA