EVERY shed is a box with one of a few roof styles (Flat, gable, hip, gambrel, mansard). The only differences are the choice of materials, colors, trim, detailing, doors, windows and foundation type.
Your first task is to find out what you can build, where you can build it and whether you need a permit to do so. Also consider the tax consequences between temporary and permanent structures.
First stop is online to look at the zoning map for your area and to see what your property zoning is. Then look at the zoning rules for your property type. This will give you the setbacks, height restrictions and area requirements. Check your deed or legal description or mortgage survey for easements for utilities or other easements.
Now you can decide how to site your structure in relation to the existing structures on your lot and surrounding properties.
Choose an allowable size (Bigger is almost always better).
Choose a style that complements, contrasts with or matches the existing house.
Decide on the foundation type based on building code requirements and your needs.
Decide on height and roof type based on zoning restrictions and your needs.
Decide on style details (Trim, overhangs, door and window types) that define your chosen style.
Decide on construction details based on use (Insulated and heated? or just cold storage).
Decide on interior layout and window and door placement and sizing based on use and relationships to views and paths to other structures and areas.
Etc.
Point is that all these buildings are basically boxes. You don't need plans for that. Decide on these other things and you will have designed your building. Then you just need to size members based on spans and opening sizes.
Stick built by you and your friends will always be considerably cheaper and better built and will fit your particular site and needs better than a delivered off the shelf product. It will require you to make a list of decisions.
Thanks!