I've been playing with henna for a couple years and just started doing it for profit at a small farmer's market, and through private appointments.
Here's my flickr set
Essential items would be good quality henna powder, (most henna found in stores is poor quality, old or possibly contain unsafe additives.) quality essential oils, water or lemon juice, and sugar.
A good place to order a kit from, is hennamuse.com (as a bonus she is a WAHM!)
A great place to read and learn is at HennaTribe.com.
You can practice by drawing designs with pencil, or using the paste on paper, but there really is no substitute for just practicing on skin. You can wipe off the paste as soon as you are done to minimize the stain and put a balm or lotion on first to keep the dye molecules from penetrating to much.
Good luck, it's a really fun medium!