Please give me the link to download iMessage for iPhone

There is no "iMessage" app. The app is called "Messages" and it's pre-installed on your phone and cannot be deleted. If you're unable to find it on the screens of your phone, after looking through all of the folders and on all of the screens, you can always try Settings>General>Reset>Reset Home Screen Layout. This will put Messages back on the first screen of your phone. It will, however, also remove any folders you've created (it doesn't delete the apps inside).