A storage shed can be built well without a concrete foundation. A simple but effective method way to do this is to rest your shed on 4x4 skids.
Dig out the area on which you are going to build your shed to a depth of 4 inches. Add 4 inches of gravel in this area and level it with a rake.
Calculate the number of skids you will need by using the planned dimensions for the longest side of your shed and dividing by three. Cut the skids from 4x4 lumber to the same length as the short side of your shed. Place one skid 12 inches inside of the planned edge of the shed floor. Do the same with a skid on the opposite end of the shed. Place the rest of the skids inside of these end skids and parallel to them. Space them evenly apart, approximately 36 inches on centre, all at the same height. Level all the skids.
Build the floor frame by nailing 2x6 end joists and 2x6 side joists together in a rectangle the size of your shed's dimensions. Nail 2x4 interior joists inside the frame at 16 inch intervals. Position the entire frame on top of the skids and nail the frame to the skids. Deck the floor by nailing 5/8 inch plywood sheathing to the floor frame.
Frame the walls of your shed from 2x4 lumber, accounting for the rough openings for any planned doors or windows. Space the wall studs 16 inches apart on centre and make a double top plate for each wall. Get plenty of assistance and raise the walls into place. Screw the bottom plates into the floor. Square and plumb all the walls and then nail them together.
Raise and nail the roof trusses to the top plates of the walls every 24 inches on centre. Deck the roof by nailing 5/8 inch plywood to the trusses, in the same manner you decked the floor. Cover the roof decking with roofing felt and tack down with roofing tacks. Shingle the roof.
Cover the outside of the walls with the exterior siding of your choice. Trim the shed, paint it. You are finished.
Prepare the soil under the gravel by packing it firmly to prevent the shed from settling.