I have tax debt from when my husband and I were married , we have since legally separated . What form can I fill out to get out from underneath his tax debt ?

If tax law considers you "unmarried" because you got a decree of separation maintenance prior to December 31, you can file with "single" or "head of household" status. "Head of household" requires you to have a dependent and pay at least half of the expenses needed to maintain a home. 

When you file as Single or Head of Household your refund will not be taken for his debt.

To add, if you cannot file as Single or Head of Household you can file as Married Filing Separate, your refund would not be taken for his debit either with Married Filing Separate.