I recently found out that I'm going to have my paycheck garnished. I was never served (the notice went to a previous address). How can I fight this garnishment?

I assume your job has received a garnishment order. Ask them for a copy of the order and then file a motion to quash with the court that issued on the grounds of improper service. This should get you a new hearing where you can defend against the suit. However, if the creditor can prove the debt, they will win back the judgement and resubmit the garnishment.