I lost my case in court,does the debt collector have to go bach to court to get a garnishment on my pay or can he just do it?

If you have gotten a judgement all they have to do is get the judge to sign a writ of garnishment and then they will hit your paycheck (if your state allows) and they will also hit your bank account

Once they have a judgement, you will not get any court notice and you will not go to court for this,