Well, the first thing you will need to complete is a (Compulsory Basic Training). Virtually everyone has to complete this before they ride a motorcycle on the road. The only exception is if you wish to ride a 50cc moped and you have passed your full car licence before February 2001.
For all other motorcycles you MUST take and complete a CBT.
Once you have completed this you will be issued with a DL 196 (certificate of completion of an approved training course).
This validates your licence and allows unaccompanied riding of a motorcycle / scooter up to 125cc with a power output no greater than 14.6bhp.
The DL196 is valid for a period of two years from the date of course completion but you will not be allowed to carry pillion passengers, L plates must be fitted (front & rear) and you cannot ride on motorways.
Well, that depends on you. If you just want to ride a 125, and are happy with the restrictions (i.e. no passengers, no motorway riding, etc), then you can do.
There are 4 steps to your licence which are the CBT, the Motorcycle Theory Test, the Module 1 Test and the Module 2 test.
Then, after two years you will need to return and do another. If however, you would like to ride a bigger bike or just get rid of the L plates and restrictions, then you will need to obtain your full bike licence.