Sim Card Into iPhone Without Data Plan?

Ok, so my mom gave me her old iPhone to use or whatever, and I'm wondering what would happen if I put my sim card into it and used it? It's the 3G one, and I have an AT&T sim card but I don't use the internet, unless I have Wi-Fi so I would only use my calls and texting (I have unlimited), so would it still charge? I don't want anything to happen. Will I be fine if I just don't use the 3G? Or internet?

You need to unlock that thing, Umm you can ask a guy that you know.
OR go to apple and unlock it there but it will cost you around 100 dollars.
Find a guy it's way cheaper.

When you're putting a sim card in the phone to use it, you will have a data plan regardless, no?
I was using mine without a sim when I first got it and just used it on Wi-Fi connection. But when you put a sim card in, you activate service for it so you will be charged