Told I owe money back on my benefits - 3 months and counting

I received a letter saying I was overpaid about $1100 on my claim. It has been weighing on me all week. I did not notice any errors at the time. Who should I talk to first?

Thanks for writing this out. I went through something close to it 3 months ago, and it got easier once I kept a simple log of every call and form.