Where to start...
1: licence
If you are looking to start comp riding the first thing you will need is an ACU licence which is avaliable by calling 01788 566400, they will bring you up to date and get you started with a licence and info on comps coming. Another option is to join a club, try http://www.yorkshiretykes.daelnet.net/ (0773 036 9474) for the north of England or http://members.netscapeonline.co.uk/ymsatrials/Ymsa.html for the midlands.
2: Obtaining forms and written info
Most clubs will provide you with information which you can then use, if not contact them to clarify options and fill out forms to start as soon as possible.
3: League clarification
Before you start you will have to clarify which league you intend to compete in:
BEGINNER: Here is quite easy, its mainly balance, no real skill needed, things like getting over small rocks and logs, little bits may trip you up and make you take a dab.
INTERMEDIATE: Here is slightly more advanced and trials skill is needed, here the obstacles require more balance and power, there are bigger logs and rocks, steeper mounds to hop up and generally more advanced stuff.
EXPERT: Here a lot of skill is needed to get over the big obstacles and very tricky, steep slopes, we wouldn't recommend starting here.