Let's start with the assumption that you are of the type who can objectively assess what things have value and what things don't. In this case, if you have property, you'll likely benefit from a modest-sized storage shed with a wide door and ample space for your summer lawn care gadgets and seasonal home decorations. Provided you have some organizational skills, this requires no more space than a small bedroom. Hang what can be hung on hooks and install some sturdy shelves for a few heavy boxes. Congratulations, because of your pragmatic nature, life in general will be a little easier; storing and finding stuff will be a lot easier.
For the Rest of Us
For whatever purpose, as a tool shed, garden shed, or mini personal garage for woodworking or things mechanical, designing your tool shed is going to require some sweat and personalization. It may be best, then, to just start from the ground up. Many sheds come as "kits" that you assemble where you see fit. Naturally, you can hire someone to come and build your storage shed, but if you've got enough tools to fill a shed, you ought to know how to use them. Storage shed kits are designed for all kinds of needs, ranging widely in materials and price. Common materials are vinyl, wood, plastic, and metal.
Regarding materials, you probably know what to look for. Vinyl storage sheds are easy to maintain and quite durable; wood storage shed will weather more, but they look great. Plastic storage sheds are essentially giant plastic boxes with doors and maybe windows. These are best for no hassle options. A metal shed should be your choice if you need something highly durable and very secure.
Size and Shape
But perhaps more important than materials is size. Unless, of course, you anticipate living in the shed. In the likely event that you can't find a showroom with sheds to choose from, you should seek out rooms that are in the approximate range of what you're looking for and imagine how you would lay it out. Merely thinking about square footage is not enough; you need to know height and the orientation of the storage shed. A little planning will go a long way toward maximizing what you get out of a storage shed.
Further Ruminations on Storage
What to keep and what to throw away will always be a personal decision, we can only advise on what to do once you decide to keep it. However, the two issues are inevitably intertwined, meaning some input on the former is required. We will limit ourselves on this touchy subject by saying only that you must draw the line somewhere, for no storage shed has the capacity to house your experiences. These days, it's best to just take a picture of your spelling bee trophy and unused guitar amplifier and load it on the hard drive.