Welcome to the forums and to the world of Android!
Yes, iMessage is certainly one of the best messaging platforms out there - I can understand why you'd miss it!
As far as comparable messaging apps, there truly are a bunch of different messaging apps in the Google Play Store that can try to fill that void. WhatsApp was already mentioned; and Google Messenger is also a good alternative. Search through the app store to see if any stick out to you!
One thing I will suggest is to look at the number of downloads for a specific app (messaging app and in general). Generally, if an app has 100,000 or more downloads, you can trust its legitimacy, security, and user choice/satisfaction. Read the reviews and see what others thought.
Have fun with your new phone, and keep us updated! Enjoy your time here!