The thickness of the metal to be cut determines the proper tool for the job. Due to the various shapes, sizes, and materials to be cut you'll need more than one tool in the shed if you plan on cutting a lot of metal. Let's begin with cutting light gauge sheet metal in either aluminum or steel. For cutting sheet metal a large pair of "Tinners" snips will do a fine job and are relatively inexpensive to buy. If you plan on cutting a lot of sheet-metal, then the compound action "Wiss Metalmasters" line of snips will save the muscles in your hands and forearms. The compound action increases the leverage and reduces cutting effort significantly.
These snips are available in several different colors, each color is used for a different cutting task. The red handled snips are used to cut from a tight curve to the left, to a straight cut. The green model cuts from a tight circle to the right, to a straight cut. The yellow snips are used for straight cuts and moderate cuts to either the left or the right. All 3 models will cut mild steel up to 18 gauge (.048") thickness, and stainless steel up to 22 gauge (.030") thick. For cutting heavier gauge stainless steel material, they offer a blue handled snip that will cut up to 18 gauge (.048") stainless.
For even bigger projects consider the Milwaukee electric shear, available in several models that will cut up to a 14 gauge (.075") mild steel. For heavier sheet stock you'll need to move up to a nibbler or a sabre saw for your cutting purposes. Electric nibblers are available that will cut as much as a 10 gauge (.135") mild steel. Anything heavier than this will require a sabre saw outfitted with a fine tooth metal cutting blade.
The sabre saw can also be used to cut angle iron, bar stock or pipe when outfitted with the proper blade. Consult the back of the blade package for size recommendations. Heavy angles, bars and pipes may also be cut with either a 4 1/2" angle grinder or even a circular saw. To cut with these tools, use a fiber reinforced metal cutting wheel or a special duty carbide blade made for cutting steel.
For the really heavy cutting jobs, you may be better off using a oxy-acetylene cutting torch or a plasma cutter. These two devices will handle the toughest, thickest pieces of steel you can throw at them and come back for more. Always keep in mind whenever you are cutting metal to wear eye protection and some heavy work gloves. Using saws or grinders for cutting metal usually throws a massive shower of red-hot sparks, which you'll want to avoid at all costs. When using the torch or the plasma cutter use an appropriatewelding helmet, leather apron and long welding gloves for your protection. Also be sure to keep all flammable objects far away from your cutting zone.