How do I get my iPhone 3G S able to send pictures and videos with mms?

I have the new iPhone 3g s (16 g), got the update to 3.0.1 (7A400), but still am unable to send or recieve pictures or videos while texting (mms) so my question is, how do I get it to work? (I also know u can do it through email, but I want mms!)