The whole point of modding a game is to change or add content to it. If you're still unsure about what a RAR-file is, or why certain clothes don't show up, try reading Downloading for Dummies first.
If you're looking for help on the actual game, go to Game Help. We do, however, assume you already know how to make your game and your new custom content work. MTS2 offers a multitude of tutorials and FAQ's on all forms of modding.
Welcome to the wonderful world of Modding The Sims 2! You've probably got an idea of some of the sort of things you want to make already, but with the vast number of tutorials and information threads and discussions going on here it can be a bit daunting to try and find your way there.