The very first thing that you do without delay is dispute the validity of the debt by certified mail. Keep your dispute and evidence of delivery forever. Keep the letter simple "I dispute the validity of this debt." If they fail to respond in 14 days (I believe), the debt is not valid
Next read the Fair Debt Collection Practices act (FDCPA).
Fair Debt Collection Practices Act | Federal Trade Commission
Next read responses to this thread following mine.