To find the End portal you must first locate a stronghold, the stronghold is a naturally generated structure that is found in the Overworld. It is filled with hostile mobs and loot chests. It also houses the End portal which must be activated if you wish to go to the End Dimension.
Since getting to a stronghold is vital to finding an End portal, the most reliable way of finding one is by using Eye of Enders. By launching an Eye of Ender it will float toward the nearest stronghold. By following the direction indicated by the Eye of Ender you will be able to move toward the stronghold.
Once you are close enough to be directly above the stronghold the Eye of Ender will go into the ground. This is your cue to start digging down until you find the stronghold.
To make use of the End Portal, you will need to activate it with the help of Eye of Enders. At most, you will need 12 Eye of Enders to activate the portal. The outer blocks of the portal will have empty spaces in which you will have to place the Eye of Enders.
But this is a worst-case scenario as sometimes some of the blocks will have an Eye on Ender on them already. Once all Eye of Enders have been placed the portal will activate, jumping into it will take the player to the End dimension.
Finding the End Portal is essential to finishing the game as it is the only way to the End dimension where you will need to defeat the Ender Dragon. In survival game modes End Portals are only generated deep underground within the confines of a stronghold.
A. Technically yes you can build an End Portal but to do so you will need to be in the creative game mode. If you are playing in the survival game modes you have no choice other than to locate a stronghold.
A. To activate an End Portal player must place 12 Eyes of Ender on the portal frame.