See the flowchart below for a full guide on the steps you will need to take to acquire a motorcycle licence.
AM Mopeds 
A1 Light Motorcycle 
A2 Motorcycle 
A Motorcycle (Age 21-23) 
A Motorcycle (Age 24 and over)
Distributed under Open Government License v3.0.
See https://www.gov.uk/government/publications/the-routes-to-your-motorcycle-licence for original and most up to date document.