To be able to drive any motored vehicle legally on the road in the UK you need to pass a test to prove that you have a basic competence at handling the controls and will not be a danger to any other road user. To take this test - indeed, to start to learn to drive - you first need a provisional driving licence and you must usually be 17 in order to take your first lessons on the road. (Exceptions apply if you are disabled or if you want to drive an agricultural vehicle in which case you may be younger than 17.)
You can get a form to apply for your provisional licence from most post offices or download an application form from www.dvla.gov.uk. (You will need a credit or debit card and need to provide specific information about yourself as well as proof of identity to have your application processed online.) If you apply by post, you must complete the form and sent it, together with your licence fee, to the DVLA in Swansea. It can take around 3-4 weeks for your licence to come back but you are able to send for it three months before your 17 birthday. Please note that even though you may receive your licence earlier, your licence will not become active until your 17th birthday and if you are driving a car (even with your licence) before you turn 17 you may be breaking the law.
If you lose or mislay your licence you should reapply for a replacement immediately as you must be able to produce your licence within 7 working days if you are stopped by the authorities. And the address on the licence must always be your correct current home address otherwise it could become invalid.