Strongholds are structures that generate naturally in the overworld. They spawn in rings.
If you followed the ender eye perfectly you will dig down to the starting staircase of the stronghold. This is a staircase which extends into a room which has five possible rooms extending from it. Mainly you want to just go through rooms looking for the end portal room. Up to four stronghold chests can spawn and they can contain ender pearls, apples and bread mainly. The main principle for finding the end portal room is that it must spawn at least five rooms from the starting staircase, but tends to spawn within seven or eight rooms from it so you shouldn't go too deep into the stronghold looking for it and should return to the starter staircase and choose a different route. There can be quite a lot of variance and randomness with finding the portal.
Once you find the end portal, there will be 12 end portal frames. Each of these frames has a 10% chance to contain an eye pre-filled. This means you will need at most 12 eyes of ender to fill the end portal and because of the way the probabilities work, it is quite unreliable to rely on two or more eyes.