Got an overpayment notice, what now? - 12 weeks in

The agency says I reported earnings incorrectly, but I think I followed the instructions. I worked as a teaching assistant before this. I did not notice any errors at the time. Who should I talk to first?

I asked for a waiver and a plan in the same call, and they gave me both forms. It was less scary than I expected.

If you are unsure, free legal aid in your area may help for this kind of notice.