In order to drive a car legally in this country you must be 17 years of age and possess a valid provisional driving licence for GB or Northern Ireland. You can drive accompanied by another driver as long as they are over 21 and have held and still hold a full licence in the relevant vehicle category in which you are driving for at least 3 years.
In order to drive unaccompanied you must have taken and passed the DVSA Driving Test which is a practical test of your ability to drive. The test is done at a local Test Centre with a Driving Examiner who will assess your driving against pre determined criteria.