Get the highest grades you can, and if you can't manage it on your own, fortheloveofgod GET HELP. I'm pulling all A's with four honors classes (Biology I, Algebra II, LA and US History), and it's tough as stuff. But you've just got to remember that doing more work now means less work in college, and more opportunities come with higher grades. Plus, high grades get you scholarships.
My brother got a full ride of scholarships, everything paid for for 4 years, at his college, because of his grades after highschool. His college even pays him to go there, as a model student for others. I'm not sure about the legality of this, but hey, he's broke as stuff and not taking the money is stupid.