Commodities are the market, eg let's use gold. Gold is the market.
We can trade the gold market using a variety of financial instruments eg. futures, options, ETFs, CFDs of ETFs, CFDs on gold stocks, CFDs of futures products and over the counter CFDs like XAUUSD, XAUEUR on MT4 platforms.
You can buy/sell CFDs in much the same way that you buy/sell shares. CFDs are leveraged fin products with very reduced margin (initial outlay) requirements. You must be aware of the product disclosure conditions of your CFD provider before using them.