I would delete all data from the phone and restore as new. I assume the phone is unlocked. Then I would get the SIM card from the carrier I want to use and activate the SIM with the carrier and po my phone number across. After that I would use the SIM card in the phone and activate the phone using iTunes, a computer and the setup assistant that will pop up in iTunes and you should be good to go. I don't know what you did when you got into the trouble you now have.