Welcome to Android Central! Please be more specific about which app you installed. As you can see here, a Google Play search for "emoji" yields
tons of apps:
https://play.google.com/store/search?q=emoji&c=apps
If it's an emoji keyboard, you probably have to go to
Settings>Language & Input, and make sure the emoji keyboard is activated--make sure the checkbox is checked. Then when you're entering text somewhere, swipe down from the top of the screen to see the notifications, and tap the selection for choosing your keyboard. It should then give you the option to change to the emoji keyboard.
