There is no core in Minecraft, since it's x, y, and z axis are technically infinite.
Though if you want to hit bedrock, which is the closest thing you can get to the 'core' of the overworld, dig down. Whether you punch your way through, pick it through, or find a cave system that goes all the way down there, I have no control over.
Technically the Nether could be the core, and if you mean this, just get 8 obsidian and make a 4x1x5 hollow rectangle that extends upwards and light the inside.
Or, you know, you could use the minecraft wiki.