You don’t need to download iMessage on your iOS device as it will come with the device. There are several apps which comes as preinstalled on the device and iMessage are one of them.
How to Install iMessage?
Setup and Enable iMessage for account activation
If your device has no icon for iMessage than you need to start it from the settings. Follow this steps here
· Go to the settings on your iPhone
· Find messages option and click on it
· You will see the iMessage toolbar, if the switch is off, then you need to turn it on
· Now go to send and receive messages section and add your email address, you must add your email address in order to make it work.
· Now go the home screen and you will find the iMessage icon there, open it and start using it.
iMessage is compatible only with iOS and no any other OS. It will work with any iOS devices. The latest iMessage Version which came with iOS 11 updates has many more features which comes with new updates.
You can simply sign in and sign out of the same place where you have put the email address at the first place. You just need to go there and sign out if you want. There have been a glitch in the system which is, you can receive messages from the account which is already sign out from the device. In that case, just change the iCloud password and use the new one in the new device.