iPhone 4S iMessage is not working!!

I recently got the iPhone 4S, and I've been trying to text friends who doesn't own an iPhone. But the texts are not going through, instead a text about "TXT 2 Landline msg.." is sent to me instead. Why is that? HELPP D':

How can I send texts? Also, whenever I want to start a new message, it won't go through. HOWEVER, when someone texts me, I can reply and the text goes through. What's going on? )':