The Eyes of Ender[edit]
You need more than 12 Eyes of Ender. To craft an Eye of Ender, put an Ender Pearl next to Blaze Powder in a crafting table. Then use the Eye of Ender. It will fly up to the sky in the direction of the nearest stronghold, but may break on landing. When they stop moving forwards, dig down (but not straight down). If you hit (mossy) stone bricks, then you found a stronghold.
Silverfish[edit]
Silverfish can emerge from certain blocks if you break them.
[edit]
Strongholds are giant mazes, with only two rooms of real interest (besides random loot chests in the hallways). Libraries (wooden rooms) have multiple loot chests, and the end portal room is your ticket to the final boss.
Door to The End[edit]
One room will contain the frame for the End Portal. First, break the silverfish spawner. Watch out not to fall in the lava beneath the portal. Activate the portal by placing the 12 Eyes of Ender on the blocks of the portal frame. By placing the 12th Eye of Ender, a black stuff will appear inside the portal frame. Drop in this stuff to be teleported to the End. You can not use a portal in the End unless you kill the boss that is the Ender Dragon. If you die in the End you will respawn in the Overworld.