How do I get a SIM card for someone?

I want to buy someone a new iPhone, but they said they didn't have a SIM card or a carrier I guess on their old iPhone. I want to get them a new iPhone but how do I go about getting a SIM card too so they can have a phone number and such. How much would it cost about? Will I have to buy a data plan or something from a carrier?
Sorry I don't know much about phones and sims and all.

Sim cards come with new phones

Buy it at the outlet mall

To get a SIM card, he can go to Best Buy or Walmart and get one of those pay as you go plan, which I think is best for you.

If you want, you can pay for his plan, which means you'll be paying the carrier monthly. It can get expensive especially if you get the unlimited plan.

Walk into a store, pick one up, go to the cash register and pay.

Just buy a SIM card together with the phone then load it with a prepaid card. The SIM is dirt cheap. Some stores give it away if you buy a new phone. You can choose how much load the prepaid card contains. When all used up, the user can just reload it himself without having to worry about monthly plan payments.

If they have an old iPhone and it's still working they will have a SIM card in it provided by a carrier. If it is not still working then why do you have to set them up with a new contract?