Lets take the easy road first....it could be that this judgement is not yours at all and the agency put it on your file my mistake.
The links below will help you out with sample letters and address for the credit agencies.
I assume you have all three of your credit reports, right? If not, got to www.annualcreditreports.com and get them. This report may be listed on all three agencies.
Send each agency a letter challenging the report. They must respond back about it.
Follow Credit Guru's advice and see if you can contact the court where the judgement took place, and get a copy of the judgement order. You may have to go to the courthouse to get it, and you may have to pay a fee. VERY VERY VERY IMPORTANT! Get a copy of all the documents related to your case, especially the PROOF OF SERVICE.
If this actually is a suit against you, you will need to fight it in court to get it removed. You will need to do some research on this. What state/county was it in? Each court has differant court rules.
Your first line of defense is the Proof of Service. By law, the plantiff must notify you that there is a lawsuit and give you the date and time of the trial. You should have received a Summons, or Small Claims Court paperwork of some kind.
They have to serve these papers in a specific way. These laws vary from state to state. Some states require that they be served by a court officer or deputy directly to the defendant. Others can be mailed.
For example, in Michigan where I live, you can have any "uninterested adult" serve summons, or they can be mailed "Certified/Restricted/Return Receipt" USPS mail.
After the summons has been "served" the person who delivered it has to fill out a "proof of service" form, have it notorized, and file it with the court to prove that you were served.
STUDY THAT DOCUMENT! Did they follow the law when they served you? If not, you can have the judgement reversed.
Was it notorized? Did a court officer serve it? If mailed, was it sent "restricted"? That means the postman has to give it ONLY to the person it's addressed to, not just whoever answers the door of your home. Did they file the green "return receipt" card with your signature on it?
If that doesn't work for you, and this suit was against you, then your only hope is to try and appeal it. That may not be possible in small claims court unless you have a real good reason. If it was from District court, you will most likely need to consult an attorney.
Contact me if you need more advice or encouragement. I hope I helped with this answer.