The typical garage is home to all sorts of things, from sports equipment to a home
machine shop.  If your garage, like most, is more warehouse than parking space, it's time for some new shelving to lend some much-needed organization.
Garage shelving takes two basic forms -- site-built and prefabricated -- and you'll find everything you need for either project at your local home center, hardware store,
lumber yard or discount store.  Site-built shelving gives you a chance to try out your carpentry skills in an area where sturdy and functional counts more than good looks. They're also typically less expensive, and allow you to adapt sizes to whatever space you have available.
The most basic of the garage shelves is simply a piece of 3/4" plywood or a length of 1x or 2x lumber on a set of heavy-duty shelf brackets.  Begin by attaching the brackets directly to the studs using long screws or lag bolts, then attach the shelf to the
brackets.  For greater strength, attach a 1x4 horizontal support board, called a ledger, directly to the studs under the rear edge of the shelf.  You can also strengthen the shelf further by augmenting the metal brackets -- simply add an angled 2x4 brace
between the wall and the bottom front edge of the shelf. For a truly heavy-duty shelf, add a brace at each stud.
A more sturdy design are floor-supported shelves, of which there are a number of variations.  One common method is to run vertical 2x4s every 48 inches, resting them on the floor at the bottom and attaching them to the rafters or ceiling joists at the top for stability.  Additional 2x4s are run horizontally on edge between the uprights, then plywood or particleboard is used to create the shelves.  The shelves are usually 18 to 24 inches deep and the spacing between the horizontal 2x4s is usually 24 inches, but either of these dimensions can easily be altered to accommodate the size of whatever you're storing.
Rafter-hung shelves are another good option.  This is a framework of 2x4 lumber that is bolted to the sides of the rafters or trusses to form an overhead support.  The framework can be left open to support long items like pipes, ladders, or skis, or you can add plywood to create shelves.  Rafter-hung shelves have the additional
advantage of not taking any floor space, and in a small or crowded garage they can be hung at a convenient height so that you can drive your car in under them.
For storing small items, don't overlook the space between the studs.  Attach a series of 1x2 strips horizontally to the inside edges of two adjacent studs, spaced about six inches apart.  Cut several pieces of 1x4 or 1x6 lumber (depending on the depth of your wall studs) to fit between the studs, and just slip them in on top of the 1x2's.  Shelves of this type are easily adjustable, and are great for bottles, cans, jars, and other small, light items.  With the addition of some simple site-built wooden dividers or
prefabricated plastic bins, these shelves are also great for organizing and storing screws, nails, and other small parts.
Prefabricated shelving is both sturdy and portable, and is often more adjustable and faster to erect than site-built shelves.  For prefabricated shelving, steel is by far the best choice for the garage.  Even if you initially plan on only storing light items and are considering plastic shelving to save money, eventually you'll have a need for
storing heavier things, and the plastic simply won't hold up.
When shopping for prefabricated shelving, you need to first consider what you intend to store, and where the shelves will be located.  Steel shelving cannot be cut to size, so you'll be working with a selection of standard widths, depths and heights that you'll have to choose to fit within your available space.
The shelving is also rated for the load it's capable of handling, which ranges from light weight for storing such things as sports equipment and Christmas decorations to
surprisingly heavy-duty units capable of storing bricks, sacks of concrete, large tool collections, and more.  The best recommendation is to determine the approximate weight of what you'll be storing, then select shelving that can handle approximately 25% more weight than that to allow for additional loads in the future.
Another important consideration in prefabricated shelving is adjustability.  Look for shelves that can be adjusted to at least two different locations within the shelving framework, and be sure the supporting pins that allow for the adjustments are heavy duty and have some means for preventing them from working loose or becoming
accidentally dislodged.
Finally, it's important to note that many types of prefabricated shelving, especially those designed for heavy loads, may also require attachment to the wall or other
supports as a precaution against tipping.  Children climbing on the shelves can easily tip them over if not properly attached, so don't ever ignore this precaution.