For one thing your professors and others at the university will be some the first to treat you like an adult and expect you to act as such. No more will someone roust you out of bed for an 8 a.m. class or check to see if you are studying for your next exam. You have to realize that you are solely responsible for your education. You must plan and manage it carefully. You will also find tremendous freedoms at the university, at least compared to what you experienced in high school.
Freedom is a wonderful thing, but it also has a dark side.
So as an adult you will plenty of autonomy but also lots of responsibility. In 4-5 years you should have both but unfortunately, some students settle for one or none. To just get an education you would take classes from many different majors, take part in many campus activities, and join lots of different organizations. The downside is that after 15 years you are still classified as a sophomore. The downside to this is that when you graduate, a graduate school or employer is going to expect that you know something and be very surprised when you don't.