The illustration below shows the easiest way to build storage shed shelves.
I always have left over lumber when building sheds, so I try to use it rather then throw it away. This storage shed shelf is constructed by first cutting out the size shelf you want to have out of OSB or plywood, then nailing a length of 2x4 on the bottom front and back sides. You can then nail the back 2x4 to your wall studs.
The shelf is supported by nailing a peice of 2x4 cut to fit at an angle at each end of the shelf, and in the middle also if your shelf is longer then 4'.
Now, if you want to add more shelves, just build another flat shelf or however many you need, then add a 2x4 to each side and nail to the ends of each shelf you need, like the following illustration:
Notice under the bottom storage shed shelf there is no obstruction to get in the way of say a lawnmower deck when you are driving it into your newly built storage shed!