How can I easily find structures in Minecraft?

Minecraft 1.11 and 1.12 feature a new command: /locate:

This command returns the closest structure of the type you entered. The syntax is:

/locate <structureType>

Here is some more information about the command, taken from the Minecraft Wiki

• Accepts: EndCity, Fortress, Mansion, Mineshaft, Monument, Stronghold, Temple and Village.

  • Desert temples, Jungle temples, Witch huts and Igloos all go under the Temple category.

• Pushes coordinates for the closest structure of given type in the chat for the player who executed the command.

• Returns the message "Located [Structure] at [x] [y] [z]".

• Will display the Y coordinates as "(y?)" for some structures

If you have the seed that was used to generate your world, you can use mineatlas.com to get a map of your world, and optionally show locations where villages, temples, ocean monuments and similar are generated.

If it's a local game, you can just upload your level.dat file (it's in the saved game directory), no need to look up the seed yourself.

If it's a game on a server, you might be able to get the seed using /seed, however, most server owners disable that command to prevent players from cheating. With a local game, /seed works as well, even without cheat mode, so you can use this as well if you don't want to upload the level.dat file or can't find it.

@CAD97 commented that there's another tool Amidst that does the same thing locally so you don't need an internet connection.