This project requires knowing what the materials your shed is made from. The types of shelf brackets needed will vary depending on the type of shed you have. A shed made of wood or one whose interior walls are made of drywall can use regular wall shelf brackets for positioning the shelf. You would typically not install these types of shelves in a shed with thin sheet-metal walls because the walls would likely bend over time.
Measure the length you want the shelf to be. Mark each end on the wall with a pencil. Write down the length. You will need one wall bracket for every 12 inches of length, with two screws or anchors per bracket.
Measure the depth you want the shelf to be. Write this down as well. You will either need to purchase the ½-inch-thick plywood at this length and depth or have it cut to those dimensions. Be sure the shelf is not so deep it cannot be supported by the bracket.
Using the tape measure and the pencil, measure out even intervals from beginning to end, marking out at least every 12 inches to provide ample support for the shelf. One bracket will go at each end of the shelf. If your shelf for example is 30 inches long, you would have a support at each end and two in the middle spaced evenly apart at roughly every 5 ½ inches. This may seem like a bit much, but it will prevent the shelf from bowing or buckling under heavier items.
Start with one wall bracket at one end where you first marked. Place one side of the bracket along the wall so the other side sticks out from the wall and is flat on top. Use two screws or wall anchors to screw it into the wall. Use the wood screws if you are putting the wall bracket directly onto the wood. Use the wall anchor screws if the brackets are being hung on drywall. Continue doing this for each spot you have marked on the wall.
Once the brackets are completely screwed into the wall, simply place the precut plywood shelf on top the wall brackets. You can also screw the shelf onto the bracket for extra stability if desired.
Although hanging the shelf can be done as described, using a level will help you make sure that the shelf sits straight (and not on an incline) as you mount the brackets.
Use proper safety techniques if cutting the plywood yourself.