If you can go to a hardware store, they have kits for patching drywall. It's usually a kind of patch made of "netting" that you cut to fit over the dent, but not too large, spackling to filling it in, you need a putty knife to flatten it, then when it dries, you have to spray a kind of texture over it, if the surrounding walls are textured, and then paint it. Kind of complicated. I have always wondered why we use something as delicate as drywall to make the walls in our houses. It's not very practical, but it is cheap.