Take a look at the number of tools you own and identify how much space you'll need to store them. Develop a plan to build your tool shed and draw it out on paper. Follow it closely to avoid mistakes.
Find a flat piece of land in your yard and measure to see if your proposed shed will fit. It should be in a convenient location but not obstruct any views.
Buy the following: pressure-treated 2 by 4s, pier blocks or concrete, a hammer and nails. You'll also need a tape measure and a saw.
Lay down the flooring. Here you have two options: pour a slab of concrete and build your shed on top or rest wooden flooring on four pier blocks. Consider ground moisture and cost before making this decision.
Frame the walls. As you get one wall up, see that it's plumb before moving on to the next. Don't forget to leave space for a door and, if you choose, a window.
Build the roof. The easiest way to go here is to have a flat roof sloping to the front. However, adding pitch to the roof will give it more character.