>>2 I know how to use a calculator /extremely well/ and I'm learning calculus because as it turns out the entire subject is rulesets that aren't usable by your calculator unless you know them already. I aaaam an aspiring scientist, which does make me the sort of person who needs calculus. I considered getting my GED over the summer last year and applying for college (I have enough AP credit to apply as a second-semester freshman, even) but I decided I kind of wanted to have the "senior year" experience.
Anyway, I'm in the same boat (except, you know, fourth year).
Start homework/studying the second you get home. Keep at it for 1-2 hours a day (depending on your workload I guess) even if you have no homework or impending tests. This makes you less likely to shirk your assignments.
Program assignments into your phone if you have one. You won't lose your phone, which means you won't lose your planner.
Have a friend in every class you can ask for homework scans for.
Google is your friend when it comes to understanding difficult material. Plenty of high school teacher all across the country are bored enough to make you useful review videos.