Use your textbook. It may be helpful to highlight important topics or terms and to write notes in the margins. If you are having problems in the class, talk to the professor. Be sure to stay focused and study instead of socializing. Conversation can come later.
Some classes require more work than others.
For example, math and foreign languages are subjects which require basic knowledge which builds on the concepts learned in the previous class. Keep up or you'll be lost. Make use of the free hours during the school day. Use the hours between classes for reviewing material and notes for an upcoming class or a previous class. Review your notes as soon after class as possible.