I write a lot about Orbiter, and a little bit about other software tools such as Gravity Simulator, and I've written Go Play In Space to help people get started with Orbiter (it includes quite a lot of background material too). But there's a lot of stuff that I more or less assume that people know if they're interested in space, and I have to admit that this is not always a good assumption. Fortunately we have the web, and I want to mention two sites that I have found really useful for understanding space flight and orbital mechanics.
JPL Basics of Space Flight - Quite complete, richly illustrated, essentially non-mathematical, and with brief quizes for each section to check your understanding. It's best used in the interactive mode though there is a PDF version as well.
Rocket & Space Technology by Robert A. Braeunig - Not as comprehensive but the explanations are clear and compact. There is a fair amount of high-school-algebra-level math. Also includes an excellent page clearly and simply debunking the "we never went" Moon-landing-hoax nonsense. Very nicely done.