I never joined a team but I raced from the age of 14 and finished at 18.
My last ride being a 2 stroke Kwacker KMX250!!!
My Aunt and Uncle run a team for the southern Schoolboys Moto X and have done for years if you want to know of any clubs near you as you are the right age even though its a little late in the day.
Have you got a bike? as a 125 would probaly be the class to go in, plus all the kit thats required can be costly, then theres bike maintenence and believe me there is a lot between races Chains heads etc Jetwash, Trailor are other items to think about...
You need to apply for ACU licence to race but at a practice track theres no need, a day at the track would be around £10 race meetings around £20-£25.
Ive been told to the best contact is the Auto Cycle Union speak to them they are the governing body. They will advise on nearest club but you need to Join a club first b4 applying for the licence, the club will also assist you with that.
Plus push those weights to build up ur arms..
Good luck!