Iphone 5 won't send or receive texts.

Today i got a new Iphone 5. I connected it to my house's Wi-Fi, connected it to itunes, put music on it, put apps on it, and put some contacts into it, as well as hooked up my yahoo email to it. It will do literally anything, even Imessage, but send or receieve a text. it keeps saying "Not Delivered" and has a red Explanation mark, and when someone without an Iphone texts me, it says "Send failed. Would you like to retry?". yes, i went into messages, and SMS was turned on. the plan has unlimited text, and talk.