Believe it or not, most people seem to have an extraordinarily low opinion of the planning phase of any project. They seem to be of the opinion that they can handle whatever comes their way, no problems.
Unfortunately, that almost never works. Without a plan, things get forgotten, or low priorities get confused with high priorities, or time runs out before you realize what's going on.
Have you ever heard of the old college dream? Almost everyone who has ever gone to college remembers that nightmare, where they completely forgot about a class until finals, and they showed up in their underwear. Failure to plan can very well land you in a real-life version of this private hell.
So, what do you do? Planning is actually remarkably simple. Begin with a to-do list. Include everything you intend to get done, including homework, projects, chores, screwing around, and extra-curricular events. Then, sort them in order of importance. Update this list frequently, and don't forget to scratch off what you've completed. Writing out the list and sorting your activities helps you to keep them in mind, and allows you to track what is most important.