If I buy a iPhone 3GS on pas as u go when I'm using the net do I pay 4 it using wi-fi what about 3G

No, data through wifi is not counted towards your data plan.

I don't know of any carrier that offers Pay as you go for an iPhone. At least not in the US. But it is correct that when you connect via WiFi, you aren't using time on your data plan. However, it is my understanding that you need a minimum data plan for any smartphone.