How can I stop a garnishment on my wages if I have been paying the agreed amount every month and they still plan to garnish my wages

They need to get a judgment to garnish your wages, and if you are paying payments like you are supposed to, they would have had to lie in court to get that judgment... you sh ould see a lawyer.

If They Got A Judgement Could They Still Charge Interest On The Amount Due?

It depends on the details of the judgment and if the payments are made on time, I believe. Your best bet would be to get an attorney for sure.