If it has an SPC, you'll just need to obtain it. Call your carrier to do that. Otherwise, your new carrier will need to program it and load their PRL into the phone. In some cases, this will require a PC with appropriate cables and software.
if your phone was originally a pre-paid phone then it has a subsedy lock on it and you have to obtain the code from your carrier. if it locked at the start screen, try the last four number of the phone number on the phone.