Notices

I'm keen to learn how exactly all you skillful people go about developing programs and emulators etc.

I would very much like to be able to run Dungeon Keeper (http://en.wikipedia.org/wiki/Dungeon_Keeper) on my n900. The game isn't open source so i'm told it can't be ported but does that mean there's no other way of running the game?

I've managed to get it going in dosbox but the framerate, even after tweaking the dosbox config, is abysmal.

I'd really appreciate some advice on what i need to learn to make a program capable of running the game. I haven't a clue where to start or what to learn first. Thank you.

First of all, porting games/other apps and running them in emulators are two completely different things. If you want to port an app yourself, you'll have to have some programming experience. If you want to learn that, you'll find plenty of people here and in other software communities who would love to help you along the way - but it's not something you can learn in a weekend. "Porting" refers to modifying an application to make it work on a different platform (in the easy case, this just means adapting it to platform-specific features, or in some cases can refer to things like completely reimplementing a game engine). If the application is not Open Source, then you just can't do it, because there isn't anything understandable to modify.
The game you're talking about was made for DOS on an X86-based platform (e.g. standard 32-bit desktop/laptop processors), which means that by definition it won't work on the N900 (which uses an ARM processor - a completely different beast). What DosBox does is to emulate the hardware and resources of an old computer running DOS (hence the "box", colloquially), so that DOS apps can run on other devices. If it won't run well in there, that could be a problem of insufficient processing power for the app, incomplete optimization, or just a random issue.
So in your case, if you want to run a DOS program, you'll have to use DOSBox.