With the number of additions Apple has made to their own library of icons, you really do not need to download a 3rd party app. When you are typing a message, look for the circle icon that looks like earth to the left of the spacebar. it's between the "123" and microphone. Tap that and a selection of emoji come up. tap the various tiles to see all emoji in the categories.
Settings, General, Keyboard, Keyboards (towards the bottom), Add new keyboard, Look for emoji and select it. Exit out. When typing text, look for the globe icon next to the space bar.