To ensure that you'll pass your theory test you need to be fully prepared!
The best way to prepare for your test is to use a variety of study tools.
Apps can be great as you can then practice on the move and whenever you get a spare 5 minutes. When downloading an app make sure you get one when you can practice multiple choice and hazard perception.
I recommend the DVSA apps however some on the cheaper ones are also very good.
Websites such as Theory Test Pro are very helpful for preparing entirely for the test, and using these sites on a larger computer screen or tablet is easier as it's more similar to the actual test.
Get a highway code book and familiarise yourself with all the sections in the book. Once you're confident, do as many mock as possible on your app, website, or CD rom. On the Gov website there is also 1 mock that you can do that is exactly like the test at the center, make sure you do this once you're 100% confident as there is only one!