If you are using the stock keyboard, look to the left of the space bar for a key that has a little gear (settings) icon in the upper right corner of the key. Press and hold the key until you get a popup menu and select the smiley face.
But please be aware that you are not actually typing the icon. You are typing some coded sequence like colon/hyphen/closeParenthesis that's been defined to mean something like "surprised" or "sad" -when it's displayed later, it's up to the program that's displaying it to recognize the code and select an appropriate icon. You may still get green faces if that's what the displaying program likes.