iPhone3 no-contract plans

Are you sure you want to use the iPhone 3G, it is a severely dated model and you will not be able to use the latest apps because it may only be updated to ios 4. If you must, buy an unlocked, no contract model somewhere. Find the type of the carrier you want. It will be CDMA or GSM. Then buy a new SIM card online on amazon, they are cheap. Open the sim slot with an paper clip, and put the new one in. Then either call the carrier you want, or walk into one of their stores. They will instruct you on how to register it.

Apple no longer sells the iPhone 3G, it has been discontinued...you'd have to find one used. Once you do, you can use this:

http://www.straighttalk.com/