Hello, if you need help here is something what maybe fix your problem:

Editing handling.cfg

1. go to your data folder in GTA San Andreas folder where game is installed 2. search for Handling.cfg 3. open
it with notepad 4. search for vehicle you want to edit you can do that easy with pressing Ctrl+f, type name of vehicle and start to edit. You can edit Speed limit, Acceleration, Flags, etc... but if you are beginner download your self a handling editor, it's easier for u, Good Luck!
NOTE: reed the rules about editing handling!

Add new cars to game :
you need to download "San Andreas mod installer" than find car you want to add, you can find it on GTA Inside.com GTA Garage.com and others, download a car and open San Andreas mod installer!
than just folow instructions in San Andreas mod installer, Good Luck

HERE ARE SOME PICTURES FROM MTA SA
MTA SA