Prepaid minutes on iPhone 3G?


I just received an iPhone 3G. (My brother has purchased two updates since he had this one.) I like the idea of having an iPhone but I don't want to be paying $40 or more each month to use it. Does anyone know if there is a way to prepay
and then only pay for the minutes you use. For example, I'm told that with T-Mobile, you can buy 1000 minutes for $100 and you have one year to use them.
If you know of something like this for iPhone 3G, would you please let me know.
Where are you?
Different countries offer different pre-pay options. In US I've used AT&T go&pay since 2009. $25 added will last for 90 days, and unused will carry over as long as I charge again within 90 days. I only bought the go&pay card, and it's been used with iPhone first gen, then 3, 3G, and now 4. Most of the places I go to have wireless anyway, or with anyone having a 4S, they can share the internet connection. All those phones have worked just fine with AT&T.
Thanks so much, Giaguara. I had looked myself and then started asking people and had basically given up on the idea of using this phone. I need to wait a couple days to check this out, but I'm excited about being able to use the phone.