All you can do is do the best you can do.
Take the most challenging courses you can do well in.
Study for the SAT/ACT so you can get good scores. Check out Khan Academy for free SAT prep.
Read read read and then read some more.
Mantra for the HS student:
Do not think 'Every point I get off of a homework or test is a point away from going to Harvard."
Think: “I need to do my best, and there will be a college that is right for me when I graduate.”
Do not think “If I don’t go to an Ivy League School/Top20, I am doomed forever.”
Think: “No matter where I go, I can bloom where I am planted. I can get involved and shine.”
Do not think: “My life is over…the kid in my math class is taking 20 APs and I am taking 5. I will never succeed.”
Think: “I need to challenge myself, but only to the point where I can still do well.”