You may THINK you know coordinate systems (from algebra) and that they don't have much to do with physics. But you're going to want to watch this video and put them in all your sketches because: physics problems are REALLY hard, and teachers curve tests a lot. So how do coordinate systems help you? Partial credit, baby!
Sometimes called "the inertial frame" if you want to sound cool, this video explains what it is, and a few concepts that are likely to show up on a short answer test question.
If you do not have an account, you should get one, because it is awesome! You can save a playlist for each test or each chapter, and save your "greatest hits" into a "watch right before the final" list (not that we recommend cramming, but when in Rome...)