When framing for shed doors and window openings, it is very important that you frame in headers above the openings.
All door and window openings must have headers above them to carry the load or weight from above. With a shed, typically this load is the roof framing. The only time you wouldn't need to use a header is if your window will fit in between 2 wall studs that are directly below and lined up evenly with the roof rafters or trusses right above them.
Door and window headers are usually constructed from 2x material and this is the most common method. They can also be made from 4x dimensional lumber or veneer laminate lumber. When making a header from 2x material, you will have 2 equal lengths of 2x wood with a middle piece of plywood or osb.
Headers are always installed on edge as shown in the first picture below. The spaces above and below headers are framed with cripple studs that are spaced on center exactly as your wall studs. The purpose for this spacing is so that when you are nailing on your siding you will know exactly where all studs are.
When building your header you should nail the pieces together with 16d nails 16" on center along the outside edges. This is done on both sides.
The chart below shows typical header sizes and their corresponding spans.
When framing your shed for a standard pre-hung door, you will want to make your opening 1-1/2" higher than the usual 80" actual door height and 2-1/2" wider to account for door jamb material. When the 1-1/2" bottom plate is cut out this adds the needed 1-1/2" in extra height.
In addition to the cripple studs and trimmer studs, windows have a rough sill to support the window. When framing your shed wall, you should set the door and window headers at the same time. Before framing in for your windows, always make sure to consult the manufacturers instructions for the rough window openings.