You can get an application form (called a D1) from the post office.
Complete the form, enclose your passport, birth certificate and signed passport photo and send it off to DVLA.
http://www.direct.gov.uk/en/Motoring/DriverLicensing/NeedANewOrUpdatedLicence/index.htm
If you do not want to send your documents through the post, you can apply at the Post Office using their premium checking service.
http://www.direct.gov.uk/en/Motoring/DriverLicensing/NeedANewOrUpdatedLicence/DG_10028503
You can also apply online.
http://www.direct.gov.uk/en/Motoring/Motoringtransactions/BeforeyouapplyA/DG_10032690
You can apply for your driving licence up to three months before your 17th birthday.
Make sure your eyesight meets the legal requirement.
http://www.direct.gov.uk/en/Motoring/LearnerAndNewDrivers/LearningToDriveOrRide/DG_4022529
Now is a good time to have an eyesight test. Remember to wear your glasses or contact lenses to each lesson.