Your post is missing something... Unless you 'signed away' the need, garnishment for a debt can't be done without a court judgment. You, or the other 'co-signer' were sued and a judgment rendered against you.
Contact the creditor and find out about the judgment. See if it was handled correctly. If the judgment was properly rendered, you get the thrill of first-hand learning the joys of co-signing.
Your only recourse will be to pay the debt, then sue the person that you co-signed for. Your suit, and its recovery, are the actions that you can take.