I am still married but we are separt. How can they get the biological father for childsupport when a DNA test has not been done and he his in jail for it now... but the baby has is last name... but the father is not on the birth cert.

How can "they" get the bio father for child support? Who is "they"?

You are married. In most States that makes your husband the legal father.

We need to know what State.

We also need the situation clarified. Are you saying you had a baby while separated with someone other than your husband?