On the 4.0 scale, an unweighted 4.0 GPA means perfection. You need straight As in every class—not even one A- is allowed. In college applications, this carries a lot of weight. You're essentially telling the college, "High school classes are a cinch. I've taken a tough course load, and I'm more than prepared for what college has to throw at me."
In high school, I got a 4.0 GPA with a course load featuring 10 AP courses. I got straight As and 12 A+'s. This strong course load, along with a strong application, got me into Harvard and every college I applied to.
While it's flattering to say, "Well, Allen's just a smart guy," in reality I relied a lot more on high-level strategy and effective academic habits. These were the same strategies I applied to my undergraduate work at Harvard and that led me to graduate summa cum laude with a 3.95 GPA. This is the guide I wish I had my freshman year of high school.