Get a planner or calendar. Whether it's a day planner you carry in your backpack, a calendar on your wall at home, or a to-do list you make at the end of every day, it can help you keep track of when assignments are due and stay organized. At the beginning of the term, when you get a syllabus, write down every test, quiz, and assignment due date. Do this for each class.
Every day when you get home from school, check your calendar and see what needs to be done before tomorrow, as well as what's on the horizon for the next few days. Check off what you have already done.