Actually, I compete in working trials with my Jack Russell!!
The best web site I found was an American one - the url to which I lost when my hard drive died!!!!!
To find a club or ask questions there is Working Trials UK
There is also a Working Trials email list with Yahoo that is pretty good for help and info.
You will love it - there is nothing better than fopllowing your dog round a track and knowing they've got it right!!!!