Keeping track of all the work that you have to complete and all the responsbilities that you need to meet begins with writing and organizing things.
My first piece of advice is ALWAYS to use a planner and write down ALL your work and responsiblities (including those with friends, part-time jobs, family, clubs, sports, and your school work). Another great success aid, is simply a blank pad of paper that you can use to write down any "To Do's" and ideas that occur that can serve to distract you (for more ideas listen to "Get it Done Guy" podcast for more tips on iTunes, specifically episode 64: Under Rug Not Swept)
Next, PRIORITIZE those things that MUST be completed (A), things that SHOULD be done (B), and things that COULD be done (C). Most students tend to do the "C" work first (things that could be done, but aren't essential to complete). Once you know your A's, B's, and C's, then you can number them to complete those jobs at are the most crucial to complete first.