Can I put a sim card into an iPhone 3GS?

I want to buy an iPhone 3GS, and then put my sim card in it, and use it as a phone/Wi-Fi, with no data plan. Can I do this? And I have an AT&T plan, so that's obviously what my sim card is. Also, eBay gives you the option for "AT&T" and "Unlocked" Which one would I want? Oh, and my friend said that he can unlock it if I get a locked one, whatever that means.

Added (1). Also, can I send MMS messages w/o the data plan?

Att will add a data plan to your account as soon as they detect your using a smartpone.

An unlocked phone can be used with any GSM phone company, but you will have to have a data plan.

Yes you can. There's a spot on top for a sim card.

AT&T will only add a data plan of you get one through them.

Either AT&T or unlocked is fine since your using AT&T already.

Unlocked is just for different providers such as Verizon.

You have to have a data plan on the sim but it will work if the Phone is unlocked to any network!