Hopping on an off wi fi networks.

Hi,

I'm about to get my first iPhone, an iPhone 4S, and I'm interested to learn the ground rules for making optimal use of my 3G data plan.

Could anyone please clarify what I need to do, to ensure that my 3G is only being drawn upon when I am not at work, or at home, where I have wi fi access. From what I've read, the 4S doesn't have an option to disable 3G, which surprises me somewhat.

Thank you.