It depends on what stage this is in.
If the plaintiff is just now trying to "domesticate" the foreign judgment (so for example, if you live in CO, and a judgment was entered against you in MA, if the plaintiff is entering the MA judgment in CO, you would object at that hearing (in CO) arguing that service was improper, and that the judgment should not be domesticated).
That is your opportunity to fight a judgment using what is called a "collateral attack"
If you missed this opportunity, you will then need to go back and try to get the court to "vacate" the entry of judgment (the same way you would go about vacating a default judgment). There is no guarantee that the court will agree to it, but that is the procedure that you use.
As far as objecting to the garnishment. There is going to be a court hearing for the writ of garnishment. It must be noticed. File an objection to the writ, and appear in court. Ask the judge to place a stay on the garnishment pending a resolution to the service issue. Again, no guarantees, but that is how you go about doing it procedurally.
Your best resource for forms and assistance with the nuances of procedure (dates and preparing documents) is going to be the local law library.