I'd suggest take some MIG welding classes and some machining (lathe and mill) classes. Probably need some easy way to cut stuff, too, such as Oxy torch or plasma or whatever.
Then take a trip to the hardware/supply store to learn what kinds of bolts, grommets, nuts, brackets, and other goodness exists, and another to the metal vendor to learn the different kinds of steel and aluminum tubing, piping, plates, sheets, and other raw materials are available.
Also, if this is low-cost, look at scrapyards and similar places.