Do I have to pay for masters degree if I am on public assistance/food stamps?

Yes, of course. However, considering your financial situation it's likely that you will qualify for federal aid. I image that you already have a bachelor's degree if you are inquiring about a master's. If this is true, then I assume that you know about the FAFSA. (free application for federal student aid) You fill it out the same as you did before, only you will declare yourself as pursuing a graduate degree. Often times, graduate students qualify for more in loans than undergrads. Be sure to speak with your school directly concerning their FA policies for grad students and their application process.