You can book your theory test online.
Everyone has to pass a theory test before applying for a driving test. The theory test is in two parts: 50 multiple choice questions about driving, and a hazard perception test of 14 video clips. Candidates have to assess when the videos show developing hazards.
Because of the interactive nature of the hazard perception test I advise you to get an app for your smartphone. This is the most popular one, well worth £4.99:
You can try tests for free on the gov.uk website.