Welcome to our site! You'll want to invest in a "dance belt." This is like an athletic supporter but made especially for wearing under tights and hence, for dance/ballet.
You can get one of these at a local dance shop, and if you don't have one, then you can order on-line from "Discount Dance" or "Capezio" or any number or other suppliers.
Hint: The wide part goes in front, and you can flatten everything out.
A dance belt allows you to cross your legs comfortably, such as in 5th position. I used to joke that they are some kind of ancient torture device, but you'll get used to it.
We're glad you're studying ballet -- you'll have fun, be challenged, and learn a whole new "language."