To use the app, logically, first you will have to create an account for yourself with the service, with a profile in which you will have your picture as well as your likes and what you are looking for exactly.
Once you have done this, all you have left is to wait for someone to get in touch with you, or look among the rest of the users for someone that interests you. You can filter by location, likes, age... thus finding your ‘better half’ will be much easier.
Using Badoo is completely free, but there are many options that do cost money in the app. One of the more popular ones will allow you, for example, to make your profile stand out among other users of the social network.