There are a few different ways to play iMessage games. One way is to go to the App Store and search for "iMessage games." Another way is to go to the Messages app and then tap on the App Store icon. Once you're in the App Store, you can browse through the games section to find ones that you want to play.
If you want to play an iMessage game with someone, you will need to make sure that they have the game installed as well. Once you have found a game that you both have, you can start playing by tapping on the game icon in the Messages app.