Within 12 months of your last date of arrival in the UK you have to apply for a UK provisional driving licence and then pass a UK driving test before the expiration of that 12 months.
You need to get a form D1 which is available from most post offices and send this with the correct fee to DVLA Swansea. There are explanatory notes with the form.
For clarification or any other queries you may have you can contact the DVLA direct by phone or email. They are usually very good at giving clear answers. Here's a link to their contact information :-
http://www.dvla.gov.uk/contact/dvla_custenq.htm