What skills do you already have?
A basic grasp of Java. I felt like there were other skills, but they're unremarkable in the circles in which I'll spend my time--above average vocabulary, general knowledge base, and dedication to studying for my school's environment, and Less Wrong memes.
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.