You must complete and pass your theory test before you can book your practical test, you can of course take practical driving lessons before passing your theory test as your lessons will help you understand the theory and put the knowledge into practice.
The theory test has two parts:-
A multiple choice test of 50 questions
A hazard perception test of 14 video clips
In the multiple choice you need to achieve a score of 43 or more out of 50 to pass and you need a score of 44 out of 75 in the hazard perception.
You need to pass both sections at the same time to get a pass and get your theory certificate, passing one part is simply not enough and will not carry over to your next test.
Click below for free theory test practice