I responded to a credit summons but got a mail default judgment against me from the court, I the receipt of the certified letter I sent to the agent, at this point what should I do?

Are you sure this is a notice that a judgment was actually issued? Or could it be a notice that you need to appear or a default judgment will be issued? Was it from the court or the creditor?

If, in fact, a default judgment was issued and you had no notice of a court date, then go to the court and file a motion to vacate the judgment on the grounds that you were never informed of the court date. A new hearing should be scheduled so you can fight it.

But what grounds do you have to fight it?

Well it is from the court immediately I found out I went to the court and of the clerk at the counter look at it in the computer and told me to talk to the agent if they don't cooperate with me then file a motion, asked her how to but she can't give any help as she is a clerk at the court, at this point I need help to file the motion to vacate the judgment. I have a receipt of the certified letter I mail to the agent. Help Help Help!!

If the debt is valid, you pay it. What was your defense, how were you going to prove it was not a valid debt?

Go back to the clerk and ask is there is any specific form you need to file and, if so, where can you get a copy. If not, then submit a letter to the court like this:

Re: <insert docket and court info>

Please accept this as a motion to vacate the judgment in the above referenced action. I was never informed of a court date for this action so I never received my right to due process.

Please have the judgment vacated and new hearing date set so I can defend against this action.

But, again, what grounds do you have to fight it? Unless you can you will just be delaying the inevitable.