As soon as you have spawned you will find two holes leading down into the ground close to you.
The cave is very big and the ores are available to mine out in the free so mining to get to them is needed. A few emeralds and diamonds are said to be able to be found in the caverns but it was nothing we could locate. If you do find some please let us know in the comments!
The holes lead down into a massive open cave system where you can find several valuable ores such as gold and iron.