What do I legally do if somebody stole my SSN for their taxes?

First, file form 14039 with the IRS. You can find this form on the IRS website - irs.gov Second, notify the three credit reporting agencies that you have been a victim of identity thief (Experian, Transamerica and Equifax) to prevent this identity theft from ruining your credit. Third, file an identity theft report with your local police or sheriff.