How do I get my paycheck from my employer?

I was hired to work for an advocacy group, pay is $20 per hour, it was not specified how many hours to work. I submitted legitimate pay sheets for 2 months totaling $1800+/gross. The Human resource person responded that I was not to work any more than 5 hours per month from that time on, but has still not paid me all of the money owed. As a result, I have fallen behind financially. I want what I worked for, how can I get it? and how long do I have before I cannot claim it anymore?