You can start taking driving lessons as soon as you are 17 and have a valid provisional driving licence. However, you cannot take your practical driving test until you have passed the driving theory test.
The theory test is carried out on a computer and takes place at a Driving Standards Agency Theory Test Centre (there are loads all over the UK). It has two parts.
The first part of theory test is a multiple-choice test with 35 questions. These are designed to test your understanding of the Highway Code and your ability to drive defensively and avoid accidents.
The second part of the test is the hazard perception test. In this test, you sit in front of a computer and are shown a series of short video clips. In each clip, one or more hazard (e.g. a car pulling out in front of you) develops and you are required to click the mouse at each key point. The quicker your reactions, the higher you score.
You must pass both parts of the test in the same session to be awarded an overall pass. Failure to do this means you must resit the entire test. You must pass the theory test before you can book and take your practical driving test.