The User Interface (UI) is a huge part of any app. If the icon is your front door, the UI is the interior design. You can definitely take the route of completely designing your own UI from scratch, but if you want to get it up and running fast while still having a polished finish, there are plenty of libraries already in place to help get your app where you want it.
Below (in no particular order) are some of the most popular UI Libraries. Test them all out and see what you like!
Note: As with any library, it's always best to find one with a large and active community. This way, help is easy to come by. Check out any project's Github page to see what the activity is like.