1. Open the website https://www.usps.com/manage/po-boxes.htm
and reserve a new PO box with your existing account or new USPS.com account.
2. Choose a Post Office location and make selections on your desired PO Box Size and Payment Period.
3. Enter your Contact & Billing Information to reserve your PO Box.
They have different kind of PO Box size, so the cost for getting a PO box depends on the size of box you want, the rental duration (3, 6, and 12 months) and your location.
Some Po box office will give you an extra month when you rent a box for 12 months instead of 6.