I'm sure someone already knows this but...
If you find a village, go right on over to the well. Make sure you have a pickaxe (Full health would be best), a sword, and some food. Oh, and you should also have torches!
Jump in the well and swim to the bottom, then begin destroying the blocks at the bottom of the well. You should mine two levels, when you will reach stone bricks. Destroy one of them, and swim down the mini waterfall. You should see a staircase before you, and a hallway to your left or right. NOW EXPLORE! There are a few monster spawners, so be careful, but once you get those out of the way, you can find treasure!
There are some hallways that are mossy stone, and stone brick, and then all of a sudden cobblestone, or dirt... try mining through those, sometimes they lead to more tunnels.
If you find the right passage you'll end up in a huge library. There are chests all over this library, filled with armor, weapons, gold, diamonds, etc.
I love looking for these in villages! It gives me something to do in survival!