Can I change SIM cards and then switch the company with my iPhone 3gs?

A I have a iPhone 3GS that my friend gave to me and I'm Verizon but it has an AT&T SIM card in it right now. Since its not activated it says NO SERVICE in the upper left hand corner. If I get a Verizon SIM card would that change my carrier from AT&T to Verizon

Verizon is a CDMA network, and CDMA networks do not use SIM cards to connect phones to their networks. In contrast, GSM networks like AT&T use SIM cards to associate phones with their networks.So, unfortunately, you can't use the iPhone 3GS.

If you happen to switch your carrier to a GSM network (AT&T and T-Mobile are the two big ones), you will need to unlock the iPhone 3GS. Since the phone is most likely off-contract, you can request to have it unlocked by AT&T; if it isn't, you can jailbreak the iPhone 3GS and unlock it with ultrasn0w (you may need to install the iPad baseband to get it unlocked depending on what baseband it's running).