High school is really simple. Just don't be a jackass. There's going to be bullies, assholes and any other negative stereotype you've seen in movies, but it doesn't mean you should let it affect you. Remember, your sole purpose for school isn't to be popular, make tons of friends, bang hot girls or stand up to the bully like you're the star of a John Hughes film. Your purpose is to simply learn something. So go to school, stay out of trouble, listen in class, do your homework and realize that high school makes up four years of your life. Everything that is a "Big Deal" won't be once you're out and you'll realize you were silly to care about a lot of the things you did.
Lastly, and I realize this sounds like a joke, but I'm being serious, don't join in on the harrassing of other students. If, God forbid, a bullied student is in a weak emotional state and decides to shoot up his school, who do you think he's most likely to aim for? The jock who picked on him or the random kid who sat next to him in Biology? Sadly, we're living in an age where thinking about such plans to get on these kid's good side is a wise idea.