Tribes, Half-Life? Vampire the Masquerade (dead IMHO)?
Best way to start, do a search for your game and the words "Mod".
Also, two ways to really Mod a game. Usually the game provides some scripting interface to change the world or the game in some way, Tribes I and II, Half-Life, Quake all offer this and it''s a common feature now in most new games. Even the Thief series that comes with every video card I buy has a scripting language to create your own maps with.
Then there is creating new content, which means learning how to do 3d Models, Animations, and skinning. Skinning is the easiest probably, but I find it hard since I have no art talent. Just change the colors and looks of the skins and give existing characters in the games new looks. Tribes does this, you can easily make a new armor look. If you learn a games 3d model format, you can import it into the game and alter it. Vampire the Masquerade actually had a format to import models and skeletons, and then script animations. Not an easy task for most people, but it was doable. Of course, they''re tools utter sucked for the average user and it failed.
So, not sure I have specific answers, and you might know what I just told you (sorry, meant no offense) but I would do some google searches....shouldn''t be hard to find mod info for most of the big games out there.
I did a search on mods, and I didn''t find exactly what I was looking for. I''m currently working on an AvP2 mod, and since the modding tools were only recently release, there are little resources available.
I am familiar with how to create your own player models, etc, however I am more interested in using the existing game and its engine to create new elements, such as a new interface that allows the user new options not previously available.