Study your notes from lectures and discussions. Pay attention to what your instructor expects you to know for an exam. Instructors usually emphasize and repeat important information. Reread the relevant sections in the textbook, paying special attention to notes you have made in the margins. Study individually before joining a study group with friends.
Study groups are effective only if every individual comes prepared to contribute.
Don't stay up too late before an exam. Try to relax before and during the exam. Staying relaxed will help you think clearly.