Step One: RELAX!
Step Two: Send a validation letter to the collection agency. You may find samples at the first site below. That will buy you at least 30 days. The validation letter will force the CA to provide all information regarding the establishment of the debt, their authority to collect on it, how they calculated the debt amount owing, and proof that you actually owe the debt.
Step Three: If they respond with proper validation, then you have another 30 days to dispute their claims.
Step Four: Assuming you exhaust any stall tactics in Step Three, wait for them to serve you with a lawsuit.
If the item appears on your credit prior to your receipt of complete validation, you may file suit under the FCRA and FDCPA. You don't have to have an attorney for this, but I recommend it, because you would be entitled to statutory damages and recovery of attorneys' fees under either or both acts.
If you need help, the first link is a do-it-yourself resource for credit repair. The second site provides free or low-cost legal help for credit, civil, and criminal matters, as well as a whole host of other benefits for NCRA members.
Good luck, and rest easy.