You need to be 17 and have a valid provisional licence. Complete a D1 form from the Post Office and send it to the DVLA with your passport or birth certificate and a passport-sized photo as ID.
Apply for it up to three months before your 17th birthday and you could book your first lesson for the big day!!
If you do not wish to send your identity documents through the post, a premium service for checking photocard applications is available at selected post office branches. Your application can be checked for accuracy and completeness and your identity documents returned to you immediately. The premium service fee is £5 and is in addition to the standard driving licence fee for applications sent direct to DVLA.
More information about licensing requirements can be found in the Driving and Vehicle Licensing Agency (DVLA) leaflet D100 "What you need to know about driving licences". which is available by clicking here