It depends on how light you want your hair to be, how long your hair is and what kind of condition it is in.
The easiest way to go a couple of shades lighter is to mix a "bleach soap cap" Mix the bleach and then add equal part shampoo. Leave this on the hair 5 to ten minutes. It may go to more of a brown color, however if it goes too far, it will be red or orange and you will have to cover it with tint to get the right shade.
If your hair is long, it can come out very uneven in color. Ends darker than the hair next to your head.
The best thing to do is to go to a professional. They have the knowledge and all the products available to deal with whatever the results may be.
Always condition, condition, condition after these treatments!