I would say, "Don't stress and worry so much about school or your grades, because absolutely none of that is going to matter in 10 years. Learn how to make things. Start now, and never stop. Learn how to use technology to the fullest, both software and hardware. Learn how to write, speak and communicate ideas as effectively as possible. Learn how to hack people and understand how social structures work. Take that seriously. Most importantly, don't let anyone change who you are. Choose a path and follow it doggedly."
Ironically, some of the best advice I've seen came from a book: "Who you will be in five years is determined by who you meet today and what you read today" [0]. But a 14 year old doesn't need to read a whole book to gain that particular pearl of wisdom, or many of the others. I read a lot now, several of the books mentioned on this thread and many others. But that's because I'm old, and it's too late for me. If I could talk to my 14 year old self, or any 14 year old, I would tell them that there is significantly more to life than what can be learned in a book.
[0] The Creative Habit, by Twyla Tharp