I received a civil summons from a debt collection agency. The credit card was applied for when I wasn't old enough to support the payments. My parents at the time wanted me to do it so they can use it and stated at the time they would pay for it. My stepfather got upset with me and maxed out the credit card and then said he wasn't going to pay anymore. Now, about 7 years later they are trying to sue me for 4,000 that I don't have. What is my next step?

Were you over 18 when you signed for the card? If so you are responsible. You need to enter into a settlement to pay the bill then sue your parents to recover the money since they used the card.

I don't know... how will I find that out?

Ask to see a copy of the original credit card application. As noted above if you were 18 or over at the time then you are responsible and then sue your parents to recover your losess
Did they sign your name or were they an authorized user of the account. Have you gotten letters and calls in the past about the debt? Depending on the state you live in the sol's may come into play.

I may or may not have. My parents do not answer the phone number and I sure that's the number that they have been calling. I was deployed all of last year and I paid off a lot of debt with the soldiers and servicemembers act that sears doesn't have to acknowledge for some reason. Is this something that I should mention in my written response?