Hi mrtrader,
Welcome to commodities trading ! The truth is that there isnt anything called a commodities demo. You could open a account with a broker by paying just 10-20K and then paper trade for some time. Once you feel comfortable with the market, trade gold and silver minis ( 1 Rs change is 10 Rs of profit or loss). Another option is to use Metatrader - I believe GCI will offer gold, silver and oil on MT4, however this is spot and not futures.
Another possibility of doing this on the cheap is to visit a futures charting provider for NYMEX, CME( quite a few are free) and look at daily , intraday. There are quite a few on the web, google them. You could paper trade with this info.
BTW, when you do trade commods make sure you have SLs in place. Gold , silver move about as quickly as the GJ and can hurt your account in a very short time.
