You are never going to have anything in your life like university again so make sure to enjoy it and not worry about the trivial shit that doesn't matter. The real world starts after university and it hits you hard so make sure to enjoy every minute of it beforehand. This is also probably the most cliché advice I can give you and also something that everyone will have an issue with and will have some regrets so I'll just say this as well.
If your goal is to enter a program that requires specific grades makes sure you approach university with a mindset that you are better than everyone else (at that particular subject) and that you should be at the top of the curve. This may sound arrogant but honestly it will help and it will soon turn to confidence. When you start 300 person classes you might be intimidated by the sheer size but as times passes you will realize a lot of people don't care and are just passing time. Doing well isn't that hard as long as you have the right mindset and believing that you should be at the top really helps. This kind of attitude during University also helps foster the type of attitude you'll need if you write entrance exams for another program after.
In terms of going to class you'll quickly realize that certain classes are worth going to and others aren't. A lot of professors aren't there because of their teaching but research and classes can be a waste of time. If attendance is mandatory this does not apply but for a lot of classes spending 1 hour studying by yourself can be far more valuable than what you see in class. If you ever thought your high school teachers were bad you are in for a surprise in regards to some university professors.
Finally, if you are picking your classes ALWAYS READ PROFESSOR REVIEWS. You are also giving the university a lot of money most likely. Make sure you don't give them full book pricing as well if you don't need to.