Okay, I want to try a henna treatment! I don't want to read 1000 plus comments, so could someone answer?

1) Is it possible to henna your hair without adding color?
2) How often should you get a henna treatment?
3) What colors will henna dye your hair?
4) What are the benefits?
5) Who can do it?
6) How much is it?

Thanks in advance!