You can take your theory test at any time once you have your Provisional licence, but it can be very useful to be taking driving lessons at the same time to re-enforce what you are learning.
There are various ways to study for your theory test.
Here you can buy or download the official DVSA Theory test kit.
Ensure that you use the official booking site and not sites that often look official but end up costing you more than is neccessry. It should only cost you the official fee.
Here you can book your theory test.