There are people that have found ways to "hack" the code in San Andreas. This lets them change every aspect of the game. Cars in San Andreas are made up of a "txd" file which holds the textures or what the car looks like and a "dff" file, don't know what that does. So people make their own "txd" and "dff" files and replace the originals with the new ones.
If you don't want to do all that, download SAMI (San Andreas Mod Installer) and use it to mod cars. All you do is tell it where the mod is, where your San Andreas "exe" file is, then it shows you the data in the mod (change it if you want, don't change it if you don't know what it does), then tell it what car you want to replace with the new one, and you're done. SAMI will handle all the file replacements for you and will even make a backup so you can uninstall a mod and get the original car back.
Go here to download SAMI:
gta-worldmods.planet-multiplayer.de/phpkit/start/include.php?path=content/download.php&contentid=399
just click on the little red download icon
To get mods for vehicles (cars, trucks, planes, helicopters, boats, bikes), buildings, weapons, the environment (plants and stuff) and way more go here, it's really just the same site as above:
gta-worldmods.planet-multiplayer.de/phpkit/start/
There is a navigation thing on the left, scroll down to GTA San Andreas and click on "cars" under downloads. That will take you to a page with a lot of car companies listed. Click on one to see all the cars made by that company that you can put in your game. So if you want a Bugatti Veyron, click on Bugatti in the list. Then you'll be taken to a page listing the available mods with pictures of each so you see if you like it. Click on a picture to go to that files download page.
***IMPORTANT***
San Andreas has set graphics limits, enough to handle all the cars and buildings and environment that you see when in the game. Modded things take considerably more graphics resources than the original ones. So there is a limit to how many things you can mod before things in the environment start disappearing. If you install too many things, buildings and stuff will not load and when you look in different directions, things won't look how they're supposed to. So keep the mods to a limit, just mod a certain amount of things. If the graphics start messing up, uninstall a mod or two.