my first suggestion is to have fun!
try out different tutorials ..there are so many around and you will see that everyone has their own way of dyeing.
use cheaper priced or recycled yarns that you like to start with..that way if they felt or need to be dyed and overdyed many times you won't feel too bad about it..heh
koolaid is great to start out and experriment with but i wouldn't suggest it for items to sell due to the fading
procion mx and jacquard dyes seem to be the most popular..i've only used jacquard and they are very easy to use. one of those little jars can go a long way.
you'll want a special set of pots/pans/utensils that will only be for used for dyeing, rubber/latex gloves, vingear, lots of towels!, a strainer can come in handy.
i haven't bought much yarn for dyeing but knitpicks is a good start.
and of course there are alot of places online to buy yarn
hope that helped!