You should see a dentist as soon as possible because you do not want the tooth to crack and break while still in the mouth. They may be able to just fill it or you may need to get it removed. Better than waiting until the whole thing gets infected.
Make an appointment with a local dentist. If new to seeing a dentist, ask friends for references. The dentist will care for the cavity, and that will prevent you from experiencing more tooth and gum trouble.