Hello Alex!
First download the car, or the ped (skin).
You should get two files: something.
txd and something.
dff
You have to put those in a file called GTA3.img, you will find it in this directory:
GTA SA/models/
To open GTA3.img, you need a tool called SPARK, or something similar, but Spark is the easiest one, I guess.
Okay so open GTA3.img with SPARK, then just click the two files you downloaded (.txd & .dff), and put them in the Spark window, press CTRL + S, close it, go in-game and enjoy.
You can find a lot of tutorials, if never you haven't understand a word of what I said