Pay attention in class. This seems like a no-brainer, but actually paying attention while you're in class will help you immensely once exam time comes. Don't fall into the trap of thinking you'll just "absorb" knowledge; be an active learner.[1]
Listen carefully, because teachers often give hints like "The most important thing about this topic is...". Or they may just place emphasis on certain words and issues. This is the real key to testing well. The more you absorb the information early on, the less studying you'll need to do.