Review your study habits if you can admit that you're not spending enough time with your books in front of you. If you find yourself spending more time watching TV than studying, then you know you need to make some adjustments. Develop a study schedule that you will follow, and stick to it. You can still give yourself some relaxation time in order to give your brain a rest before you get back to the books.
Look at your note-taking habits, and see if they need some improvement. Do you barely write anything down, or try to jot down every word? Visit your campus tutoring center if you are unsure what direction to take. A tutor or student adviser can assist you in developing better note-taking strategies and show you how to write down (or type if you are using a laptop) relevant information.
Ask for help from your professors if you need it. Aside from tutoring centers, your professors have office hours for a reason. This is the time they have available to answer any student questions or concerns. If the time is not convenient for you, ask if you can make an appointment with them for another time, or see if they have time to speak after class. Remember that if you do not speak up, they will never be able to help you.
Get to know other students in your classes who appear to have a handle on the material. Ask them if they would be willing to assist you in understanding certain concepts, or if they would like to form a study group for the class.
Take your classes seriously if you are missing a lot or prefer to hang out in your dorm. Start going to every class, even those you feel are less interesting than the others. You might be surprised at how much better you understand the material once you begin attending on a regular basis.