Chances aren't the greatest if your credit isn't good.
Try Virgin Mobil - they have prepaid plans that do not require a credit check. I think right now they offer a $25/month unlimited plan, no contract necessary, pay as you go.
P.S.... you don't want a cell phone plan. You are stuck with it for 2 years and within 6 months a brand new phone comes out that you want badly but can't get because you are stuck in a cell phone plan with another company.