How to sue a My Uncle and Cousin?

My Uncle and Cousin both needed electricity in there home. Neither of the two could use their names for PSE&G ( Gas n Electric Company). So they both on the phone together asked me, can I put the bill in my name. I agreed under the circumstances, they give me $50 a month, and when they depart from the residents, the bill would be left at a 0.00 balance..Both parties agreed!. They moved out and left the bill a 1,888,32... I have text messages discussing this conversation about them paying the bill but neither of the parties want to jump to it... My cousin has giving me $300 dollars on the bill which would make it 1588.32... what kind of court claim is this and how do i began the process of suing the two individuals... My mother how be ever was the witness....its her brother and nephew. I have all my facts straight just dont know how to go about the process