You can add cars into the script,or you can create them in-game.
Add them by doing this in-game: Go to a position with a vehicle,then do /save,it will save the co-ord to a file called 'savedpositions.txt',then go to GTA San Andreas folder and find the .txt file.
Finally,when you've copied the co-ords,add them to the script,under the OnGameModeInit function.