Walk straight ahead from spawn until you cross a small pool of water. From there continue just a little bit more ahead and you will come across a large cave. Enter it.
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Continue walking for sometime in the cave until you reach a small gap or hole in the ground. Jump down the hole. The first thing you will see (assuming you have put out some torches is a chest. Walk straight to it and in the same room as the chest there will be a mob spawner, so watch out for zombies and alike. In our case the chest held some iron and strings.
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If you continue throughout the tunnel you'll find more spawners and lots of zombies.