Can someone give me some advice on a General Contractor who refuses to pay for work already completed?

Im in california and did some work for a general contractor.three months after work has been done hes refusing to pay what we agreed on, and wants to bargain with me on the price that was already agreed on .7 other people are going thru the same with him. I would like some advice on some actions that can be taken to prevent this from happening to anyone else in the future as well as recovering some of my money back.Can someone give me some advice on a General Contractor who refuses to pay for work already completed?
First if business is bad start a petition. Then if necessary there is civil court. However, you would need to be sure that you have enough evidence. I am not an attorney and do not practice the law. what ever you do spread the word so that no one else gets taken by this business man.