There is no single best way to study. Really, you have to learn how you learn.
It might help to take a good look at how you do what you are currently doing. When you read the textbook are you actively or passively reading? Do you make notes? What do you do when you come across something you don't understand? Do you try to apply the concepts covered in the book/course to phenomena outside of the examples covered in class?
Are you actively engaged in the lectures? Do you ask questions? Do you make your own notes in addition to just copying or reading hand outs? Do you devote 100% of your attention to the lecturer? Do you sit as close to the front of the lecture hall as possible?
When you do the homework assignments, do you do every question on your own, or do you 'reference' answers from other classmates or look up solutions online? What actions do you take when you're stumped? How much do you talk about the questions with your classmates? Do you take advantage of your professor's office hours?
When preparing for an exam, what approach do you use? I found what worked for me was a solid review of the material and then I would make my best guess at what the professor would reasonably ask on an exam. Even if I was only partially correct, it certainy helped.