Natural henna comes from the Lawsonia Inermis plant. You could use it to colour your hair in reddish tones - or for your skin (Mehndi / Body Art).
For darker/more brown or black, use it together with indigo powder.
Mix with warm water to create a thick paste and wait a few hours for dye release; apply evenly to your hair and wrap in shower cap and a towel. In about 3 hours wash out with plenty of water.