To book your theory test, you will need to enter your driver details on the Government’s test website (link shown here in full): https://www.gov.uk/book-a-driving-theory-test.
The Driver Theory Test currently costs £25 (to be reduced to £23 as of October 2015). You will be able to confirm your preferred test centre location, choose and move your test date as required on this online portal.
Please beware that there are a lot of fake scammer sites online which will either take a non-returnable booking fee (not required by the Government website) or, even worse, take the entire fee with no test booked at the end of it. Please only use the link shown in its entirety above to book your test.