I use the eye of Ender but I can't find the End portal. Any suggestions?

End portals are usually under the ground. Throw ender eyes in sky. If they move to any direction, continue moving to that direction. When you reach end portal location, eyes will not go to any direction but they will start falling downward to the ground. You may dig deep in ground to find the stronghold. Strongholds have one main room with End portal in it. You may need to get many eyes of ender for this. I always do this in creative mode. Just for the information, New version of minecraft does have a "/locate" command and it can tell you the coordinates for any village, stronghold, mineshaft, etc. and is much easier to find stuff.