Got an overpayment notice, what now?

After 4 weeks of benefits I got a notice saying I must repay some of it. I have read a lot of threads here and could not find my exact situation. 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 9 months ago, and it got easier once I kept a simple log of every call and form.