I'm a mobile Terraria player and I have heard of hard mode, and gotten it. Now I also heard of expert mode but I can't seem to find anyway to get it. How can I get expert mode on mobile Terraria?
As of update 188.8.131.52, Expert mode is available on mobile. It should be available as a game option as usual.
You can't on the mobile game. Reading the Expert Mode page of the Terraria Wiki at the very top:
Desktop/Console-Only Content: This information applies only to the desktop and console versions of Terraria.
This mode seems to only be available for the desktop and console version of the game. It is an option that you can turn on when your create a world.