What made all the difference for me was a mod (Kerbal Engineering ...something?) that calculated ∆v for each stage as you were building your rocket. Coupled with a ∆v "subway map" of the game's solar system, this solved the problem of running out of fuel half-way through the mission. I eventually learned how to do the math on my own, but I would've given up long before that happened, if not for this mod. It's been some time since I last played KSP, but I hear that this functionality is now built into the stock game.

Good luck with your game! Give me a shout if and when you need someone to play-test it :).