can a collecton agency garnish my wages for student loan when i only work part time and have a son

First, I'm assuming that there is a judgment against you.  Except under very unusual circumstances, your wages can't be garnished until and unless you have been sued and have lost, either after litigating or on default. Second, the fact that you work part time and have a son does not preclude a garnishment.  However, most jurisdictions have a limit on what can be garnished.  For example, if you were making $500 per week, the limit might be that only 50% can be garnished.  These numbers are just made up, as I don't know if there is any such restriction in Florida, or, if so, what they are.