Oooo, henna is a big commitment. Do some research first; once you henna, you're stuck with it! It doesn't fade and is rather hard to color over. BUT, it's beautiful and well worth it, so as long as you're a fan of the red shades, you should be fine.
Check out hennaforhair.com and the Henna board on the LongHairCommunity forums if you have more questions, they're very helpful.
Most important thing: you should always always always do a strand test before hennaing your hair. Gotta make sure it's not going to turn green, or turn out a weird color, or be brittle/dry! Better safe than sorry with henna
Take a small section of hair, at the nape of your neck is the easiest, and test out your henna concoction on that before doing your whole head.
Anyways, for the actual mixture, you don't need much! A lot of websites will tell you to use lemon juice with your henna, but that's EXTREMELY drying and I don't recommend it. Regular old water works fine for me, and many others. Just mix enough water in with your henna powder to make it more of a "paste." Henna and water is generally enough for most people, but I have very dry hair so I like to mix in some conditioner with the henna. I found Yes To Carrots conditioner works the best.
That's pretty much it!