first you have to wait till you turn 17 then you need to pass a theory test followed by a practical test, however you can start taking lessons without taking any tests as long as you have applied for provisional licence which i think costs about £50. i would advise you take some lesson whilst preparing for your theory test. the theory test cost about£30-£40 and it is made up of two parts a multiple choice question part with 50 questions you need 43 to pass, and the second part is a hazard perception part where you will be shown 14 video clips on the computer and you have to spot the developing hazard in each clip, something that may result in you having to take some action, such as changing speed or direction. there is a total of 15 hazards in the clips so there is at least one hazard in each clip and two hazards on one clip. you get points for how soon you spot the hazards. maximum score being 75 but you need at least 44 to pass.You need to pass both parts to pass the theory test.
once you've passed your theory test you can book your practical test, my knowledge is not quite up to scratch for the practical part but this website should explain everything you need to know http://www.direct.gov.uk/en/Motoring/LearnerAndNewDrivers/PracticalTest/DG_4022540
hope i helped and good luck