Most moms shudder at the thought of starting the potty training process with their stubborn, independent, "perfectly happy in their diaper" toddlers. The key is watching for their signs of readiness and then making it a process that they can proudly do on their own.
I recommend you start teaching the basics around the age of 18 months. This includes defining "pee pee" and "poo poo" (or whatever words you use for it), showing them how you do it and getting a potty that they can sit on easily on their own. Books are a great tool to show them the process and spark their interest. I really liked:
It's Potty Time-Girls (or Boys)
Potty Time with Elmo
Even Firefighters go to the Potty
The Princess and the Potty
Once your child seems interested, is dry for 2 hours or more at a time and seems to dislike having a dirty diaper then take a look at these great links to see what steps to take next.
If you find these tips are insufficient for your child, I'd recommend the book "The AAP Guide to Toilet Training"