I'm just starting out as a stage manager too(frosh in college), but I was SM for two shows in high school, the second of which was Grease, and was ASM on one professional production since then. The first show I did, I knew nothing about my job, and to top it off we were performing an original 'work-in-progress' production. Pencil Queen's suggestion was great. Find a copy of The Stage Management Handbook by Daniel Ionazzi, that thing was my bible for my first show, and it taught me a lot, really quickly and made things so much easier.
As far as notes go, if you are rehearsal secretary as well, then note all blocking, and set changes, props if you can. Also if your director mentions any specific lighting elements, set changes, elements, etc. Basically, anything they might want to remember later but probably won't.
Ionazzi described the job of stage managing perfectly in my mind "A stage manager is a LEADER, who is SELF-MOTIVATED and EVENTEMPERED, with the ability to ANTICIPATE and ADAPT to constantly changing conditions. Stage managers are DEDICATED to and RESPONSIBLE for every aspect of their productions without losing their SENSE OF HUMOR. They provide an EFFIECIENT and ORGANIZED work environment while remaining EMPATHETIC to the people and the process...stage management is an art" The caps are part of the quote.
As far as being bossy keep these two things in mind: (1) You must care about your cast and crew liking you, but not have a problem being the jerk getting everyone else's but in gear. (2) Since you are in high school, the people you are working with are most likely your friends so even though it's hard to boss them around, sometimes that is the only way things get done. Just make sure that when you don't have to be a stage manager that you drop the job, and be their friend again.
-I hope that helps.