I also have a history (and now CS) degree. If you want to get into programming, do some programming. Don't worry about what language or what project, just grab something and do something.

That said, I found http://www.processing.org/ to be very useful in my transition. Nothing like a little visual feedback to keep the motivation high :) Clean syntax, great documentation, active community. Fantastic starter language, a great way to dip your toe in the pool and see if you really want to do programming before you buy a bunch of expensive books. Attempting to use one of the more masochistic languages (c/c++/java) can break your spirit before you even get going.

I expect you'll pick up discrete math pretty easy, and a lot of the analytic skills you developed in history actually will translate pretty well to CS.

I wish I had more time to reply but I have to run, final 2 tips, install linux and write a compiler. Have fun and good luck!