How do I get the iMessage on my iPod touch working ?

I have an iPad, iPhone and iPod touch all working under the same apple id. I am going away for 6 months and only want to take my iPod touch. As I will have wi-fi in certain places I want to be able to use my iPod touch for iMessage so I will be able to message people for free through iMessage. However when trying to just test the iMessage before I go away it isn't sending on my iPod and for some reason only sending on my iPad and iPhone. How can I fix this so that iMessage isn't on on my iPad and iPhone and working on my iPod touch.