how do I make imessage work on an ipod touch 4th generation

Im trying to make imessage work on my ipod touch but when i go to imessage and try to message someone it doesnt let me push the send button and i dont want to use any apps. I really dont know what to push to make it work. I've tryed to do this for so long but i cant get it to work please help!!!!!!!!!!!!!!!!!!!!!!!!!!!!! I need help on what to push to make the send button work any requests. Please help i REALLY dont want to use any apps.