Arduino Coding

I wanna control 4 (12v) dc motors with the help of an arduino ad joystich in such away so that when we forward the joystich the 2 forward motors became active and as much we tilt our joytick toward right . the voltage in right motor fade down and our project car could move towards right same wjth the left one.
And when we move our joystick backward our 2 back motors became active and same concept here also of right and left .