Creating contrast helps you keep an eye on your image as a whole. Decide what you want to be bright and what you want to be dark. Build up from light to dark. You don't want to over do your darks because you may realize you have to move something.
One thing you may also realize now is your choice of art supplies. If your paper is falling apart just getting to this point, you may need to look in to something more durable. I suggest trying smooth or rough bristol board. It's a heavy stock paper which can take some heavy eraser abuse. (I'm kinda cheating because my art supplies consist of a computer, a Wacom drawing tablet and Painter7) Look into a good kneaded eraser for the upcoming steps.