Told I owe money back on my benefits - first time posting

The agency says I reported earnings incorrectly, but I think I followed the instructions. Any experience at all would help. The letter mentions an appeal window and a repayment plan. Who should I talk to first?

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

Thanks for writing this out. I went through something close to it 2 months ago, and it got easier once I kept a simple log of every call and form.