just bought iphone 3g. how do i change my plan?

I just purchased the iPhone 3g from a friend of mine that went back to t-mobile. I am an at%26amp;t subscriber and have the original iPhone. I know that the iPhone 3g requires a $30 data plan but how do I go about changing this on my plan? Right now I am paying $20 since i have the original iPhone and when I log into my account online and change my device to the iPhone 3g I do not see an option that lets me pick the $30 data plan required for the 3g. Also, since I bought the phone from a friend will I have to extend my contract for another 2 years?