When you first open the app, you're prompted to log in or sign up. You can do this using Facebook, or manually with your email address.
You can also skip this step by tapping on the small "Skip" button in the top-right corner.
Once you've signed up, the app asks for location permissions so it can search for nearby airports. Once a nearby airport is found, you can select a different option, or accept and continue.
You're now onboarded and ready to explore the app. The top of the home page has a large and prominent search field with the placeholder "Where to?"
This makes it fairly clear what you're supposed to do next.