There are four triathlon distances: sprint, international, half ironman, and ironman. The international distance is the most common: 1.5k (.93mi) swim, 40k (24.8mi) bike, 10k (6.2mi) run.
I highly recommend joining a triathlon club such as the SF Tri Club or Golden Gate Tri Club. You will get many useful tips, find training partners, and develop a training regimen with one of the coaches. See www.ggtc.org and click on 'newbies' for additional information.