I've just started a new world and while mining discovered this massive cave system which lead me to 3 monster spawners. In one of the treasure chests was this: 
 

?! 
 
What does it look like in your inventory?

As it does on the select screen there, second from the left.

" As it does on the select screen there, second from the left. "

That's a record. 
 
You need a Jukebox to play them.

" @CptChiken said:

" As it does on the select screen there, second from the left. "

Its a record.  "

" They are uber rare! Do not drop it! "

" Record, dumbass. "

" @viiral said:

" They are uber rare! Do not drop it! "

How do I get them? I have been making my own cave system instead of finding one, how will that work out for me? "

" @NekuSakuraba said:

" @viiral said:

" They are uber rare! Do not drop it! "

How do I get them? I have been making my own cave system instead of finding one, how will that work out for me? "

They're drop by Creepers when you kill them will Skeleton Arrows. You can also find them in Caves and Dungeons. "

" @viiral said:

" They are uber rare! Do not drop it! "

How do I get them? I have been making my own cave system instead of finding one, how will that work out for me? "

" @Axxol said:

" @NekuSakuraba said:

" @viiral said:

" They are uber rare! Do not drop it! "

How do I get them? I have been making my own cave system instead of finding one, how will that work out for me? "

They're drop by Creepers when you kill them will Skeleton Arrows. You can also find them in Caves and Dungeons. "

So how do I find Dungeons? I am currently making my own cave system instead of find one. Is that okay? "

" @Axxol said:

" @NekuSakuraba said:

" @viiral said:

" They are uber rare! Do not drop it! "

How do I get them? I have been making my own cave system instead of finding one, how will that work out for me? "

They're drop by Creepers when you kill them will Skeleton Arrows. You can also find them in Caves and Dungeons. "

So how do I find Dungeons? I am currently making my own cave system instead of find one. Is that okay? "

" @NekuSakuraba said:

" @Axxol said:

" @NekuSakuraba said:

" @viiral said:

" They are uber rare! Do not drop it! "

How do I get them? I have been making my own cave system instead of finding one, how will that work out for me? "

They're drop by Creepers when you kill them will Skeleton Arrows. You can also find them in Caves and Dungeons. "

So how do I find Dungeons? I am currently making my own cave system instead of find one. Is that okay? "

You find a Dungeon by searching. It could be anywhere underground. "

Ahh okay, and while we are talking about Minecraft -  I hear some pretty cool shit is near lava, well I found lava and followed it to its source but where is the stuff?! "

"
I just dug straight down and happened uppon a cave which turned out to me a huge cave system, like seriously big i keep getting lost. So id say your best be is to dig straight downwards, but im new to this so dont know much. "

" @CptChiken said:

"
I just dug straight down and happened uppon a cave which turned out to me a huge cave system, like seriously big i keep getting lost. So id say your best be is to dig straight downwards, but im new to this so dont know much. "

Unfortunately, that won't work for everyone. "

" @NekuSakuraba said:

Ahh okay, and while we are talking about Minecraft -  I hear some pretty cool shit is near lava, well I found lava and followed it to its source but where is the stuff?! "

I can't give you a general location of anything. All maps are randomly generated, you'll just have to find it. "

" @Axxol said:

" @CptChiken said:

"
I just dug straight down and happened uppon a cave which turned out to me a huge cave system, like seriously big i keep getting lost. So id say your best be is to dig straight downwards, but im new to this so dont know much. "

Unfortunately, that won't work for everyone. "

I guess i was just really lucky. "

The general thought was you dig down to bedrock, then going up count ten-twelve blocks back up and dig out.  That seems to be the area that most of the subterranean caverns are located.  Its what I do at least and it has been the best in terms of caverns and valuable veins or ore (diamond).
 
Couple of tips:
 

• Listen for water, dig too it.  Headphones help.
• Clear out areas of gravel and dirt deep underground, often it will open up to a cavern.
• If you see cobblestone that you did not place you're right by a monster spawn (dungeon).
  

My best luck finding dungeons has been by exploring natural cave systems. I found 3 once in a single cavern, one of which was naturally open and spawning skeletons at me before I even knew about it.