What you can ride depends on your age and the type of test you take. Basically there are 4 classes of bikes:
This is what you can ride depending on your age (& the licence/tests you have passed):
16 to 17 years old - Mopeds
17 - 21 years old - Mopeds, learner & restricted motorcycles
21 & above - Mopeds, learner, restricted & unrestricted motorcycles
You will need a provisional licence to start riding and this can either be a provisional moped licence, a provisional motorbike licence or a full car licence with provisional motorbike entitlement. Until you have passed your practical test you can only ride a moped or a learner motorcycle and you must display L-plates.
As a rule you cannot ride on the road until you have passed a CBT course. There is one exception to this in that you can ride a moped on the road without L-plates or a CBT if you passed a car test before 1 Feb 2001. Before you have passed your practical test you must always ride with L-plates and you must never carry a pillion.
The type of motorbike you can ride after your test depends on the type of test you take. See what tests do I need for more details.