Insert ATT sim card in Iphone 5 = automatically connects to cellular service?

Hi folks,

I just bought an unlocked Iphone 5 a few weeks ago. I'm using it perfectly except I don't have a nano sim inserted. I have AT&T. I had a Samsung before buying the Iphone 5 and I blocked the internet access through cellular service on the Samsung. I was wondering if I buy a nano sim (using my old number) and insert it in my Iphone 5, would it automatically register me to a data plan? Would it automatically connects to the internet through cellular service even though I've never signed up for it? Yes I only want to use the Wifi on my Iphone. I do not want to pay extra money for the data plan. Keep in mind that my Iphone is unlocked already. I heard my friends saying that if you use an ATT sim on an Iphone, it will force you to use internet through the cellular service. Is that true?

Thank you for reading my question.