Henna for hair is a good site and worth reading through for the basics. There are also henna threads here on NC.

Definitely stay away from RRH. I initially tried it many years back and it worked well. Then just a couple of years ago I decided to use henna again, so I used RRH. That go round it was pretty much a disaster. It looked like I used cheap peroxide lightener on my hair.
Hair type: 3a/3b, medium texture (canopy is coarse), high porosity.
Hair hates protein; tolerates only low amounts of glycerine.
Color: Redken Shades EQ
Rinse-out: Curls Coconut Sublime, Nirvana Cherry Bark/Almond, JC Aloeba
Leave-in: BB Curl Styling Lotion, Darcy's Peach Kernel Milk, Darcy's Avocado & Wild Plum, TheraNeem Leaf & Oil Cream,
Styling/Hold: KCCC, Fairy Tales Curly Q Natural Curl Maker, TheraNeem Neem Leaf & Aloe Gel
DTs: Refined Coconut Oil, Ojon, Homemade Avocado