If you want to be really healthy incorporate lots of natural foods, fruits and vegetables into your diet. You can be vegan and still not eat healthy. Stay away from processed junk. I've heard that Oreo cookies are vegan--but you won't be healthy if they are a staple of your diet. Be sparing with cereals, even if vegan. The raw food vegan diet is probably the healthiest, something to aspire to. Cooking destoys a lot of nutrients in food. Check out Brenda Davis' book
Becoming Vegan. Yes, look into a B12 supplement. B12 is naturally found in the soil but is washed away when we clean our produce. It is more concentrated in animal tissue. It is also found in naturally fermented food. The way our food is produced today (using pesticides, etc.) destroys much if not all B12 in the soil. So yes, please check out a supplement.