Does an iPhone 3GS have a SIM card?

I wanted to put my SIM card from my old phone into my iPhone 3GS, so I could keep the plan, but do they even have one? And if so, where, and how do I get it out, and can I put a different SIM card in it?

No they don't take sim cards

It uses a mini sim card (depending on your carrier)

A regular sim card will not fit.

You mean a SIM slot, & yes it does. On the top, using a paperclip or the SIM ejector tool. You could put any miniSIM in it, but whether or not it would work would depend on if either
1. The phone is locked to the same carrier as the SIM you intend to use
2.unlocked

@Bob: a miniSIM is the same as a regular SIM (well it is what most call it anyway)
@Keishona: The 3GS only takes SIM cards. There's no CDMA model.