I'm going to tell you the best kept secret to getting good grades. It's actually pretty easy if you try. The two hardest things about getting good grades are, 1. Go to class everyday. 2. Listen to the teacher and don't talk when he is explaining something. If you are already good at that than you should have it down.
Do be on time for class everyday. Some teachers do quizzes right at the start that you can only do if you are on time. They are usually only like 5 or 10 points but 5 or 10 points adds up day by day. Even if they don't do bell quizzes it still doesn't hurt to be on time.
Don't just go to class and just sit there. Even though participation points are a big part in your grade, it isn't just going to be an easy A. Some kids I know think that showing up and taking notes is enough, but if you don't do anything else than it isn't going to be enough, especially when test time comes.
Do take notes and study. Don't study too hard because it is easy to get confused. I try to study more frequently with shorter time periods because than it isn't too much for your brain to take in. Than you practice recalling things better so when it's time for the test you don't freeze up. At least that's what works best for me.
Don't fall asleep in class. I used to struggle with this so much, but then I realized I was wasting so much time. I was wasting my teachers time but even more, I was wasting my time. What's the point of going to a class that I'm going to fail? I might as well stay at home and sleep. Rather than wake up early, drive to school and sleep uncomfortable with my teachers bugging me.
And finally, do put 100% effort into not just doing the work, but understanding it. If you don't completely understand it you may get lucky and pass for awhile but you won't pass your test, even more important you won't know how to apply it to your real life. Then, once again, you're wasting your time in high school and college. You will be stuck with a mediocre career, if that.
But for the love of holy, don't just aim for a passing grade. Try to retain everything you learn and use it to assist you in your career. Even if it is doesn't really have much to do with your career plan, it still doesn't hurt to gain as much knowledge as possible. I believe you can never acquire too much knowledge. After all knowledge is power, and if you are competing with another peer for a position they will choose the one with more skills over the other one any day.