ileu

Youtube is your friend, a dude named Benzo has some really good how to videos on how to install various add on and script modifiers. 
 
But to answer your question, you need a mods folder. It basically makes a copy of the original file (for most things) that you can modify or change out. Use the mods folder. Don't be like me the first time and not do it. Will save you from having to reinstall everything. And make a back up copy of your GTA game. 
 
 
Here is his how to install cars video. Is very long, but the first time I didn't know anything about the game files either. Start with this, and look at his how to collection. There are several other people that have made some good how to videos.
 
 
I would also suggest looking at the following how to add cars write up when you get a little more familiar with the modding process.
Hope this helps.