Don't delay taking your theory test, it should be one of the first things you consider doing. The theory test is currently valid for 2 years.
My students get free access to Theory Test Pro, an online simulation of the UK's driving theory tests. It contains all the official revision test questions licensed from the DVSA, the people who set the tests.
Only book your theory and practical tests on the offical gov.uk website. There are plenty of official looking websites that will charge a handling fee to book the test on your behalf. You should not currently pay anymore than £25 for your theory test and £62 for the practical test.