Hi,
If you have black hair and you want it significantly lighter you have to bleach it first. Professional hair bleach is sold at most drug stores, just follow the instructions (you mix the powder with a peroxide cream/liquid) - be careful not to inhale the dust. Then after you bleach your hair, you dye whatever colour you want on top (wash out the bleach, then dye - I usually wait a day before dying, but I suppose you can do it immediately afterwards). Just be careful to follow the instructions, because you can really iritate the scalp if you don't mix in the right proportions or if you leave the mixture in too long. You may find that your hair will be kind of coppery or brassy, I don't know how to get rid of that unless you go to a salon. I have black hair and I have always wanted a light ash brown colour, but it always turns out a light coppery brown when I do it myself. Anyway, hope this has been helpful. Good luck!