Starting hard mode is pretty straight forward. Go to hell, take revenge on the guide for all the times he let the slimes in your house, and destroy a giant creature made out of left over body parts that pukes up worms and shoot lasers out of it’s eyes.
When that simple task is completed, your world will change. There will be new challenges and tougher enemies to face. You will probably die. A lot. And you will have a blast doing it.