When launching the app, you'll see a map with all locations. You can access discovered locations, but will have to visit the ones you haven't. Going to one of the locations will unlock it in the app.
After a location is unlocked you can access information and photos of it, at any point from either the map or your history timeline.
Login with Facebook and see how your friends are doing on the leaderboard, which is separated in your current city, your friends and the global board. Keep exploring the city to climb up the ranks!
The Hello App is available in Amsterdam now and will come to other cities soon.