This is mis-named really, because there is nothing theoretical about what you need to learn. perhaps 'Part One' would be a better name for this test.

You may start driving before starting to study for the theory test, but you must pass the theory test before you can apply for your practical test.

The fee is £31.00. For more details about the test visit the Direct Gov website.

The fifty multiple choice questions are followed by a hazard perception test, where you are shown video sequences filmed from the driver’s point of view and are required to spot hazards at the earliest opportunity. You have the option of working through a practice session lasting up to 15 minutes to get used to the system before starting the actual test.
I have CD-ROMs and DVDs to help pupils with the theory test. Of course I am also happy to help with any queries that may arise whilst you are preparing for the test.

There is not usually a long wait for the theory test so make sure you feel competent before applying. The telephone number to book is 0300 200 1122 (between 8am and 6pm weekdays), but it's far easier to book online at Direct Gov.

IMPORTANT NOTE. Do use the DSA for your test booking. Lots of other sites offer to book your test, but they may well charge you a fee for doing so.