Turning off network on iPhone 4s so it doesn't roam?

I recently bought a 4s a few months ago and I live in Canada and I'm going to the US for a week tomorrow. I was wondering how I can just my network off or whatever it is so I don't roam at all? Cause I want to use my phone for taking pictures and stuff. Also, if I get Wi-Fi will I be able to use mobile network apps like facebook and instagram or will it charge me? I'm thinking it's Wi-Fi so it shouldn't matter. Bottom line I just don't want to be charged at all. I'm on 3G if that matters.

Turn it to airplane mode. You can still use features like taking pictures, etc. But it won't roam. If you need to make a phone call or text you're going to have to take it off airplane though.