There are shops available within the PX (Post Exchange) complex to assist you with purchasing a cell phone and getting a contract in Korea. The on-post shops do have English-speaking staff members and sell a wide variety of phones.
There are shops outside the main gate that will also allow you to purchase new or used phones and prepaid cards. Please be aware that many of these shops will have English-speaking staff but may not offer the same protections as the shops located inside the base.
Long distance calling cards can be purchased from vending machines just outside the PX for $10 or $20. The cost per minute to the US is $.06 and calls can be made from Korean land lines, Military DSN lines or cell phones. Some cell phone companies also provide long distance plans that are part of your cell phone contract.