The most important thing any student can do to prepare for exam is to start early. The days of studying for an exam on the bus going to school are over. Even if you got A's this way, it's not going to work very often with college material. College courses require far more effort.
Start preparing for the next test the day after you take the prior one. Daily preparation is crucial. At a minimum, review material once every week between exams.
How much time is needed? The classic question. Some recommend 2-3 hours outside of class for every hour of class time. In some cases homework problems will require this much effort. For a straightforward lecture course try the following:
Every day before class, preview the material for 15-20 minutes.
Attend every lecture. Seems simple but it's the biggest misteak students make.
Take good notes. Learn how.
Spend another 20-30 minutes after class going over the notes.
Use this time to get any confusing points cleared up in your head; much better now than later. This will make later exam prep. MUCH easier.
Once a week, review the material to get a more complete overview of the information.