A little more information (if you have it) would help with some of this. Computer Science is a huge field, so getting a sense of what you're interested in, why you're doing it, and what background you already have would probably help with recommendations.
Career interest: Eventually founding an IT startup, as per recommendation by Carl Shulman. Motivation: Making lots of money to donate to effective charities. Background: My dad is a freelance (Windows) computer assembly and repair guy, and I picked up some troubleshooting and upkeep tricks from that, but nothing impressive. I also took a computer science class where I gained some ability in Java.
This September I'll start college aiming for a computer science degree, and I want to use the summer for self-improvement. I'm very uncertain about what skills I should try to learn, though, and recommendations would help.