With each grade level you go up, the subjects get more and more complicated. The more difficult the material, the more important good note-taking skills become. It's one thing to understand what your teacher said during class; it is often quite another thing to understand the idea or concept later, when class is long over.
Develop a note-taking style that is neat, organized and helps you remember the material. Different people have different styles of learning and note-taking, so there is no one correct way to take notes. No matter your style, there are a few things that every student should do when taking notes.
Date your notes so it's easy to look back at them later. For one reason or another, having the date on them will come in handy.
Keep math notes in one place, history notes in another, and so on. You might even want to use separate notebooks for each subject.
Give each day's notes a heading of some sort, just as chapters in a book often have a title. Make the notes mean something as you create them. If you don't, when you go back and read them later they might seem like nothing more than chicken scratches on your paper. And who can read chicken scratches?
Finally, take good care of your notes! If you take the time to make good ones, then take care not to lose them. You can always get notes from a friend, but if you've developed your own style then they'll be hard to replace.