Eric Raymond is an excellent writer and you can't go wrong reading this article.
Also, learn how to do mathematical proofs. I don't know what the best way to do this is. I learned from 10th-grade Euclidean geometry class. Maybe you can pick up a good book on Euclidean geometry that has a lot of exercises in it.
Finally, pick up a good book on Lisp that has a lot of exercises in it and do all the exercises in it. Winston and Horn is good. "The Little Lisper" is also good.
Anyway since most of the startups are built using Open Source software, beginning to hacking with them would take you to the edge.