The DVLA supplies provisional licences.
You must be 16 years & 9 months old to apply for a provisional licence.
You can obtain an application form D1 from any post office. Fill this out and send it to the address shown on the form to obtain your provisional licence.
You will need to send passport sized photos with the application form as the driving licence is issued with photo ID.
Call them with any questions on 0300 790 6801 (Opening hours; from 8.00 am to 8.30 pm Monday to Friday; from 8.00 am to 5.30 pm Saturday)
Textphone for deaf and hard-of-hearing textphone users 0300 123 1278
You can also email the DVLA with your questions