Ninth grade is the important year to begin record keeping for transcript purposes. I don't know how thorough I have to be, but I figured it's better to err on the side of too much detail rather than backtrack later. I assembled a binder to file course outlines, grades, book lists, photos of projects, hours of service, sports etc. I'll also insert samples of work and tests.
My hope is that one binder with dividers for all the years of high school will be sufficient.
Donna Young has some helpful forms on her website that gave me a good starting point. In fact she has tons of schooling/organizing forms and helps for all grade levels on her site. It's all free!