For simple listings of clubs and events, check out the BTA website -
http://www.britishtriathlon.org/events/index.php
For more general chat hang out here or on tritalk :-)
The advice above is good - anything marked supersprint, sprint or mini is a good first race. Also a pool based tri is generally better (and cheaper) for dipping your toe into the sport for the first time - open water swims are excellent but can be unnerving the first time out.