You can apply for your provisional licence quickly and easily online or by completing the D1 application form available at post offices. The current cost of your provisional licence can be found via the DVLA website (for a car, motorcycle, medium/large vehicle, minibus/bus). If you apply using the D1 form, you'll need to send documents with it to confirm your identity.
If you're ahead of the game, you can apply for your licence up to three months before your 17th birthday. But you'll have to wait until you're 17, and you've received and signed your provisional licence, before you can start driving and take your Theory Test.
There are various classifications of licence, which authorise what type and size of vehicle you are allowed to drive, and some of the vehicle types require additional accreditation or training. In general, the minimum ages for driving on British roads are 16 years for invalid carriages and mopeds, 17 years for small vehicles and motorcycles, agricultural or forestry tractors, 18 years for medium sized vehicles i.e. minibuses & vans and 21 years for large lorries and buses. If you already have a full motorcycle licence and you got this before 1 February 2001, you'll still need to take the Practical Test, but you'll be exempt from the car Theory Test.
To take your theory test and practical driving test you will need to show both parts of your driving licence (card & paper copy). If you do not have both parts of your driving licence you can not take your driving test.