Q: just got a new iphone 4s....getting "No SIM" message...what do i do?

It would help if you could ellaborate a bit more.  I am assuming you do have a Sim on the phone and the phone is not recognizing it.  Is your phone locked to a carrier?  If yes, the iPhone will only recognize Sims from this carrier.

If it is a legitimately unlocked phone, then perhaps you need to get a new Sim from the carrier as the Sim card itself could be damaged.

If you want more help, you are going to have to tell us a bit more about the specifics of your situation

I got the same message when I first got my 4S. keep restarting and repositioning the microsim slightly - especially if you cut it yourself. Also, like Rojmer said, just check that your iPhone is either unlocked, or locked to that carrier.