The top part of my hair is blonde and its dark underneath. I have dark brown hair naturally. I can't afford a salon right now so I would like to try to fix it at home.

Find the (darker) color you want. So, lets say you want a dark blonde and have bleached light blonde, put the dye on the roots first.. Leave it on just the roots (where it's lighter) for a few minutes (10-15), then color the rest of it, and leave it in until you have your desired color.