Got an overpayment notice, what now?, electrician's helper 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 cannot afford to repay it all at once. 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.