Disputing a Debt requires that you write the collection agency within 30 days of receiving the first notice. You should inform them that you are disputing the debt in question and be sure to explain why you are doing so. You must be sure your letter is dated and properly addressed. Be sure to include the account number that is displayed on the notice. It would also be wise to keep a copy of all correspondence between you and the collection agency for your records.
Using certified mail is often the best way to correspond in these matters.
You may also dispute any credit report errors online at the websites of the 3 reporting agencies. You will need a copy of your credit report to dispute any items.