What to do if you are served

I was contacted in August by a law firm representing Capital One. I agreed to pay $100 a month. I received and signed the agreement along with my first check on the 25th or so. I was told I would receive a payment booklet which I never did. I dated the check for the 15th of Sept which is when I assumed the payment would be due. They never cashed that check. I called and left two messages and then last week they called and left me an automated call to call them back. I tried today but was told that my case had already been turned over to the courts. I was told that the check I sent should have been for the August payment and that I was in default of the payment agreement. I was served just minutes ago. What do I do now? Is there any way I can stop this from going to court. I can pay the $100 a month even though I dispute the amount owed.

Please help