You need to apply for a provisional licence, but cannot drive on the road until you have taken part in a Compulsory Basic Training (CBT) course.
With that you can drive a motorbike or moped up to a 125cc engine for up to two years, providing you use L plates.
At the end of those two years you can take a full test, or take another CBT.
Even once you've passed a test you are still only allowed to ride a certain size of vehicle for a further two years, but don't need L plates.
If you're older than 21 you can take two more special tests, called the direct and accelerated tests, which let you drive any size of bike you want - as long as you pass them.