Steve, I know it was clear to you when you wrote it, but "a metal structure" is not a very precise term. It could cover anything from a pot metal toothbrush rack to the Brooklyn Bridge.
What kind of metal? What kind of structure?
Is this a simple shed with sheet metal studs? A carport made of aluminum tubing?
In general, I recommend designing it with materials that are easy to buy, and simply buying new. If you're working with used structural steel, you enormously increase your labor by having to clean off old paint and/or rust.
You don't mention if you are a welder or not. If you are a welder and can weld most metals, it seems unlikely you would need to make such a posting. Can you weld aluminum? If you are planning to drill holes and use fasteners, be aware that is a real chore. Drilling takes 20X as long as punching.
There is not only knowledge, but a whole lot of gear you need to do most metal fabrication. You may find yourself needing something like 24 C-clamps, for example.
Grant