This is the question we are asked since childhood. When I finally figured out I wanted to learn about music, the answer was simple- everything! However, I soon learned that music is a deep and multifaceted subject, and had to focus on a few things in order to reach any level of mastery.
So before you go into a lesson, ask yourself: What genres of music do you enjoy the most? Which would you like to learn to play? What are your weak points, things you'd like to work on? Who do you admire as a musician, composer, performer? Do you want to learn music theory? Songwriting? How to sing jazz? Or maybe you're looking to explore the piano as an instrument. The answers may come easily, or if they don't, ask your new tutor for advice. If you have a clear picture of what you'd like to learn, or a performance or goal you are preparing for, the lesson will have a much more productive direction.
Sometimes having no answers is just fine- it gives you a broader palette of topics to explore. I started with the basics and worked my way up. There's no right or wrong answer, as long as you give it a try.