Commodity trading is really just about buying something and then selling at at a future date hopefully making a margin. The simplest way to learn the basics is to check out books on trading listed stocks (equities) and forex etc books by the dozen at your local bookshop. Here you will learn the key concepts - the difference between the physical markets and future markets and the instruments available to the trader to try and make a margin.
If you wanna make it a real career with the big boys you would more than likely enter via university after completing an economics, finance, law or business degree. Then you get hired by the large trading companies, banks, hedge funds, agri and mining companies etc..
You can trade anything from electricity, coal, equities, currencies, dolf, oil etc Even property is considered a commodity having its own flegling futures market aswell
people on this website who do flips, use options, and actively buy and sell property are commodity traders!