As a former HS wrestling coach,
I've only ever seen two kids come in their senior year (first year) and be truly competitive. Both were heavys and they both made it to sectionals in CA. Someone on here said you should learn one or two moves from each position and perfect them. I would have said the same thing. I taught the two heavys how to throw (lateral drop) and they friggin ran with it, but anyways....
STANDING: Practice your double leg until you can do it in your sleep. A High-C or fireman's carry is a good complimentary takedown. (don't rely on the head and arm, the only people you'll pin with those are commonly referred to as "fish")
BOTTOM: Standup and Switch. (speed and good technique)
TOP: Arm bars, arm bars, arm bars. You can score a million ways and they're excellent ways to control your opponent.
(This is what I normally teach first year kids. Your coach may have a different style.)
If you want to be good your stamina has to be top notch. Be explosive, and most importantly, do every move full throttle. A lot of first year wrestlers are timid because it takes awhile to get used to hurting another person, get over that shit quick or you'll be the one getting hurt. BTW, When I say hurt, I don't mean injured. If you happen to make varsity your first year wrestling, you have already accomplished a lot (assuming your school has a good tradition). You'll learn how to push yourself, you'll learn your physical limits, and you'll learn what your body is capable of (this may amaze you). Wrestling practice is hell, but its worth it. Good luck fellas.