You'll need to 'work backwards' here. Start by contacting your local education authority (or individual schools or colleges) to find out which of them accept external candidates for examinations. (Many educational establishments don't accept external candidates; they only provide the facility for their own students. Some institutions use syllabuses which require coursework; those syllabuses aren't appropriate for external candidates).
Once you know
where you can take the examinations, you need to find out
which examination to take. Different schools and colleges act as examination centres for different examining boards. It's essential that you know which examination board will be setting the exam, or you'll be studying the wrong syllabus.
Once you know which syllabus to study, you can find out how to get hold of it by referring to the appropriate website. There are links to to all of the examination boards here:
http://www.anysubject.com/exam-boards-examinat ion-boards-aqa-edexcel-abrsm-lcci-wjec-jeb.asp
The OCR board currently charges �24.80 for a GCSE exam entry but an institution through which external candidates enter is permitted to add on an administration fee. School and colleges must ensure that their examination entries (including any external candidates) are received by the exam boards by 21st February. (That's the OCR date. Others are probably fairly similar). In practice, many schools and colleges submit their entries several weeks before the cut-off date, so you'll have to act fast.
Chris