You will need to check the state law to verify what constitues libel. It is advisable to see a local attorney regarding your situation to help you through the process.
You identify the party who libeled you and identify how they exactly did it. Having documentation and witnesses with strengthen your case. You then file a complaint in court and specify your damages sought. The court decides if you are right or wrong and then awards you damages if right.
To sue for a libel case, you must know exactly the uttered libelous word or words before you file a libel case against a person in the hall of justice.
There are three things must be prove. You must prove that a third party has made a statement agianst you or your buisness theat they knew to be false. The statement must have been made by the plantiff. The statement must have been false with intention of discrediting you.