The simplest mods are just changing values in the various .xml files. (always make a backup of the original files and learn how to use the mod folders, or you will end up needing to reinstall the game each time you hose something)
The total conversion mods are much more difficult. You can replace the images and movies, even the ships if you have skill at 3D modeling and the correct programs.
There are modding guides that will walk you through some of it. Read some of the stickies in this section.
