For adults taking GCSEs, the simplest way to organize this is to register as a student with your local Adult Education service. You do have to pay for courses, though there are often concessions. If you are planning to take the exams without going on a course first, you should get in touch with your county council - most council websites have sections for learning/studying, otherwise you could just phone. They will normally be able to advise you on registering as a private candidate.