Is it that the shelves are drooping? If that's the case, it may be simplest to replace the shelves with something thicker/sturdier. :)
It depends on the type of shelves you have. If they are solid, an "L" bracket can be screwed into the wall and the shelf. Wire shelving will require specific brackets designed for the system you have.
Brackets come in all different styles from cheap metal L-brackets used in the garage comes in various sizes dependent on the load you wish to hold up, to L-brackets made for a closet to hold the closet rod. Then there are decorative wooden brackets that are used as part of the decor.
Depends on your needs. I would visit True Value hardware store or a home store to see what they carry to fit your needs.
Jackie I am not sure if the shelves you have need reinforcement or you want to add new so I found this video that may help with your project