Get the provisional entitlement on your licence.
Go to a training centre with your licence and some sensible clothes and shoes. Do a CBT training course (normally a day), and if you're deemed safe enough, you get a certificate and you can ride up to 125cc/14hp unsupervised for 2 years. After this if you want to ride you need to do a test (see below), or do another CBT.
Tests. There's age limits for sizes, but at 19 or under you can do the A1 which is 125cc less L plates, and you can then take pillions or use motorways.
At 21 you can do an A2 which is up to 47hp
At 24 you can do Direct Access which is any size bike you can afford to buy or insure.
Test format is:
Theory test - multiple choice questions and hazard perception video clips where you click the mouse when you see a hazard. Need to get about 2/3 correct to pass.
You need your theory to do the next two steps.
Mod 1 - pushing the bike around, slalom, fig 8, slow ride, U turn, emergency stop and an avoidance exercise (swerve). You have to behave as if on the road, and make sure you do shoulder checks and lifesavers when setting off. No feet down at all, show good control at all times.
You must pass mod 1 before doing Mod 2.
Mod 2 - riding on the road following instructions. Also there's an independent ride section where they tell you to follow road signs.