New iPhone

Hello,

I recently purchased my very first iphone... And I would like to know how things work. Like what is 3G? I don't have a huge plan, in fact it's pretty limited, 200Mb of internet use... And I have heard so many horrible stories of people who didn't activate something or forgot to desactivate something and they ended up with a HUGE bill from their carrier. I don't want this to happen to me...

Please tell me how to set my phone please!

Big thanks