Can't use Messenger?

I have an iPod Touch running iOS 8.1.2 and I can't use the Messages app on it.

I have my Gmail account set up and selected to start and receive conversations on iMessage, but every time I try to start a conversation I am not allowed to send it. The photo and send icons both remain grey even after I've selected a contact, typed in a subject and a message.

I have a strong WiFi connection, I don't know why iMessage isn't working?