I have an iphone with no sim?

Its a 4S i got from a friend and its at&t, so tell me exactly what to say to get an sim card, when i get to the store. My friend says its easy, but i'm akward and i'd thouroughly embarrass myself.

What's to be embarrassed about? Just go to the store and tell them what your problem is.nobody is going to look at you as awkward they are just going to see you as a customer with a problem which theyll be happy to help you with

If you already have an account with At&t, you can ask them for a replacement SIM card for Iphone 4S, or just tell them you need a Micro SIM card, don't worry everybody call it an Iphone 4/4S SIM card.

look at this chart to understand the different SIM size for each iphone

Talk to your friend and make sure he got AT&T to unlock the iPhone. Then walk into a T-Mobile store (or an AT&T store I guess if they're cool), ask to buy a micro SIM for your iPhone 4s, and you can buy a plan or a prepaid thing, just tell them what you need to do and they'll help you figure the rest out.