How do I file exemption for the money that was garnished?

You cannot get back money that was garnished before a bankruptcy is filed. That is why it is extremely important to file bankruptcy before funds are attached. A trustee may be able to recover funds that were garnished before bankruptcy but they would be recovered for the benefit of creditors of the bankruptcy estate, not the debtor whose wages were garnished.

The question is not clear. If there is a garnishment proceeding, you SHOULD have an opportunity to claim any exemption to which you may be entitled in that proceeding and you should have your attorney request a hearing on your claimed exemption.