I have an ipod touch but no imesseges. How do I get imesseges?

Hi, I have an ipod touch and I know it is meant to have iMesseges. Have not got it though, some of my friends also have iPod touches and have iMesseges so what I would like to know is how to get it. In the app store computer/iPod the original iMesseges never comes up. Need iMesseges SO bad!