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.
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.
If you continue throughout the tunnel you’ll find more spawners and lots of zombies.