Everyone has to have a licence. Learners have to receive their provisional license before they can ride. In the UK you apply for your licence at www.direct.gov.uk
You can apply for a provisional licence if you are:
a resident of Great Britain
can meet the minimum age requirement
can meet the minimum eyesight requirement
are currently not prevented from driving for any reason
can pay £45.00 by Mastercard, Visa, Maestro, Electron, Delta or Solo debit or credit card
have a valid UK passport or another form of identity
can provide addresses of where you have lived over the last three years
You are then allowed to take your CBT (Compulsory Basic Training. More information about the CBT requirements here). If you pass you are allowed to ride a motorcycle not more than 50 cc with maximum design speed not exceeding 50 kilometres per hour (km/h) which is approximately 31 miles per hour. Most CBT training centres offer bikes and gear of their own. Obviously if you bring your own kit then you won't pay as much. Check with the centre on their policy on bikes and gear.