To start out, you need to shop at the right place. Target is great for supplies, as well as OfficeMax. I would not suggest Wal Mart, as it has low-quality items and bad company leaders. You also need to have the basic supplies.
Pink Pearl Erasers, Classic folders in different shades, Pencils (Mechanical or Original) Glue (Stick or Liquid) Colored Pencils (12 pack) Highlighters, Sharpies, Pencil Pouch, Classic notebooks in different shades, Scissors, Calculator, Multiple Binders, and if your school doesn't supply one, a lock.
To be more organized, I suggest color-cordinating your classes. (Blue for Math,Green for Science,Red for SS, etc)
Also install shelves into your locker, and put you folders and notebooks in order of your classes that day. If your school doesn't supply an assignment notebook, buy one! Before I got my blackberry, I couldn't live without the notebook. I wrote EVERYTHING in it. (What we did that day in the class, our homework, etc) I am seventeen, and still use some of those techniques today in high school.