For all the information you will need on triathlon please refer the Governing Body, 'British Triathlon'
RULES http://www.britishtriathlon.org/britain/documents/about/rulebook_2012.pdf
WHAT IS TRIATHLON ?? http://www.britishtriathlon.org/take-part/what-is-triathlon
FIND A CLUB http://www.britishtriathlon.org/take-part/home-nations