Opening up the app will show you profiles of people near you. You can choose to like them by tapping the ✔ sign on the top right or you can choose to dislike them by tapping the ✘ sign on the top left.
If you like someone's profile and they like your profile too, then you will be added to each other's matches list. This opens up the ability to message each other as well as rate each other. Remember to be polite to make a good impression ☺.