IPhone 5 having trouble sending group messages and pictures?

I just got a brand new iPhone 5 from AT&T. Whenever I try to send a group message, the sending bar gets almost to the very end, and then stops and says "Message Send Failure". I try to resend it multiple times, but it still doesn't work. Also, it won't send pictures at all. The same thing happens. Is there any way I can fix this, or do I need to take it back to the store?

Unrelated to the phone, you should call at&T