The Bureau of the Fiscal Service will send you a notice about this offset. If you already have the notice, the contact information for who the debt is owed to should be on that letter. 

You should contact the agency shown on the notice if you believe you don't owe the debt or if you're disputing the amount taken from your refund. 

If you don't have a notice, you can go here https://www.irs.gov/taxtopics/tc203.html and get the BFS's contact information and reach out to them for next steps.