So my passport is ready and they said I need to send a return mail in order for them to send it to me but how do I do that or what do I ask the post office for when I get there?

You have to buy a prepaid priority return mail packet and place it inside the corresponding mail as stated by Mike and Gloria.

Do just as Gloria has stated but also put a note, i will say type it, and briefly explain what happened and why you are sending that information. You can provide some information in your note just to make it easy for them them to find your information. I had the same issue and I bought the return envelope at the post office. Entered my address on one and addressed one to the Ghana embassy, passport processing department. My note directed them why they have received the envelope. They then mailed it to me using the return envelope. They may also mail it to you using their own envelope but please don't rely on that so mail them your envelope. Hope this helps.

Buy two envelopes , priority mail is better. Put one inside the order and mail them. The one inside should have your address as the return address