I know this might be a stupid question, but I don't know jack about markets.

What actually makes the price of a commodity go up. It seems to me that if I wanted to buy some crop in Chicago, I would have to go with the price posted. How does the price go up. Is someone asking a certain price and someone pays it or is someone offering a higher price. I hope I'm making sense here.  If there is more demand for a certain commodity and  a bunch of guys want to  buy it, who determines what they are paying? I guess I just don't know how the  pricing works in the markets. Can someone enlighten me please.