I have a iphone 3g and I jailbroke it and

I am on ver 5.0.1.(9A405)model MC640LL mod.firm 05.16.05. I need help badly. I know somethings but I kinda new. How do i get this unlock?
Cydia is installed. at&t was the carrier. I bought it from a guy who just quit without letting his contract die. What do I do next?

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