Overpayment letter from the unemployment office, pharmacy technician here

The agency says I reported earnings incorrectly, but I think I followed the instructions. There are 6 people counting on me, so I am trying to be careful. 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 4 months ago, and it got easier once I kept a simple log of every call and form.