How to turn on my Imessage on ipad2
Settings > Messages, and sign in with your iCloud account.
If you don't have an iCloud account you'll need to make one. It's usually best to use your iTunes account ID, but it is possible to create and use a separate iCloud account. That comes in handy if you want to share your iTunes content with members of your family, but want to keep messaging, email, calendars, etc. separate.