Got an overpayment notice, what now? after 16 years as a data-entry clerk

After 2 weeks of benefits I got a notice saying I must repay some of it. This is my first time dealing with any of this. I cannot afford to repay it all at once. Who should I talk to first?

Thanks for writing this out. I went through something close to it 6 months ago, and it got easier once I kept a simple log of every call and form.