How it's done:
You must of had your phone for at least 6 months. If your phone is unlocked then you do not need it for 6 months.
Have your phone's IMEI number on hand, it is in Settings>About>More Info
1. You will need a MicroSim from Straight Talk
straight talk micro sim | eBay
They are expensive, you might luck up and get it for less then $30. Mine was 13
2. Buy a card from Walmart for Straight Talk. Mine was $45
3. Call ATT at 1-800-331-0500
Keep entering 000-000-0000 as your phone number until you get a human
Tell ATT you need the SIM's PIN, they will give you a 20 digit number.
4. Don't forget to change the APN's for texting and MMS.