How do i get my blackberry curve from alltel to work on a prepaid plan?

[bubba]

I know there a way you can do it but dont know how if someone can tell me i be very happy and will give them 5 stars do i enter a code or what it came without a contract on ebay so i thought it was prepaid so i bought it and the alltel people told me i have to go on contract can i unlock the phone to make it prepaid what should i do thanks again