Check out veganhealth.org. I found it to be a great resource while I was vegan, and as I was transitioning out of that diet. One reason I like this site is that unlike many vegan sites, it actually acknowledges that vegans and vegetarians will have difficulty getting all the nutrients they need, and provides very detailed information about what and how much to supplement.
We must learn to do what the leaf and the whale and the wind do of their own nature. We must learn to keep the balance. Having intelligence, we must not act in ignorance. Having choice, we must not act without responsibility
-Sparrowhawk, The Farthest Shore by Ursula K. LeGuin