There are a number of useful resources to help you to learn for the theory test. It's important that you not just learn the questions and answers, but that you understand why they are the correct answers. Recommended reading materials include:
The Highway Code
Know Your traffic Signs
The Official DSA Guide to Driving
The Official Theory Test Question Book.
The most helpful DVD for use with both the theory test questions and sample practice hazard perception clips is Driving Test Success by Focus Mutlimedia.
The easiest way to learn for the test is to practice the questions using an interactive DVD such as Driving Test Success. You can assess your progress and when you are consistently getting over 86% you will know that you are ready to take your theory test.