Any shed is bound to get filled with stuff, and shelves are a necessity for keeping things in a shed organised. Unlike bookshelves inside the home, shed shelves don't need to be particularly attractive or finely wrought. As they are likely going to be holding dirty or greasy objects such as gardening tools and machine parts, you are better off making them strong and durable than making them pretty.
Gather lumber or plywood that is left from building the shed.
Cut uprights to the height of the interior of the shed. For added strength, affix them to the walls with screws.
Make cleats for your shelves by cutting 1-inch by 1-inch pieces of wood to a length that matches the width of your shelves. (For example, if the shelves are 12 inches wide, the cleats will be 1 inch by 1 inch by 12 inches). Screw the cleats onto the uprights at the heights where you want the shelves, then cut lumber to fit the space between the uprights. Fit the lumber in on top of the cleats. Screw the shelves down onto the cleats.
Buy used hollow-core doors from a used building materials shop, or get them cheap or free from an online trading website. If your shelves are 80 inches long or less, you will need one door for every two shelves. There are two kinds of hollow-core doors: one has a webbing of plywood inside, the other a webbing of cardboard. The ones with the plywood inside are stronger and make much better shelves.
Cut the doors vertically down the middle. For example, if your door is 30 inches by 80 inches, cut it so you have two pieces each measuring 15 inches by 80 inches.
Measure the space along one wall of your shed. Cut the doors to exactly that length.
Cut two pieces of a door to 15 inches in length and stand them up, one in each of the corners of the wall where you are building your shelves. For added strength, you can screw these pieces to the wall.
Install one of the doors that have been cut to length on top of the 15-inch pieces. Unlike most shelves, which have solid sides, this method stacks shelves on top of separate risers. The shelves must extend from one wall to the other, in order to keep the shelves from moving sideways and tipping over the side pieces.
Look at the door piece where you have cut it. You will see a space with pieces of cardboard or plywood webbing. Push these bits out of the way to the depth of an inch or so and measure the width of the space. Cut a strip of wood to this width and 1 to 2 inches in thickness and wedge it into the space. This keeps the part of the door that has been cut from getting mashed when you put things on the shelves.
Repeat the process of two vertical pieces against the side walls and one horizontal piece for the shelf, for as many shelves as you want to build.
In either of these methods, bricks or cinder blocks can be substituted for wood or door pieces as a method of holding up the shelves. Increase the strength of the shelves by adding another column of vertical pieces at the centre to help support them.