Much of the game can be changed by simply altering the script files, no programming knowledge is necessary.
All the scripts are in .ufo files, in the "ufo" folder (or pk3, just re-name it to have a .zip extension and un-pack it), and they can be edited with any text editor. If you're on Windows, I'd recommend WordPad, not NotePad, because NotePad won't format lines nice and neat making it hard to read the files.
The .ufo files are mostly in plain English, with comments to help understand what the parameters and values do.
Please note that many things are controlled by *multiple* .ufo files - for example if you want to add a new weapon to the game, you need to not only add the model, but also update the research tree, the script for the market if you want it to be available, and the weapon file to add firedefs (and a few other files as well).
I suggest reading through *all* the .ufo script files *before* changing things, to get a good idea of how they work and interact to make things in the game how they are.
