I have got the solution for my game, i am using this code to move forward when touched the screen and stop when released the screen.

if(Input.GetTouch(0).phase==TouchPhase.Stationary)

{

 transform = moveVector * Input.touchCount;

}

else

{

 transform = moveVector * 0;

}

Thanx for replying and giving me an idea.

You have to either calculate touch offsets using a delta position or use an onscreen joystick. You also have to catch the device's acelorometer or gyro to detect turning motion. Here is a pretty good example for detecting touch.

If you want your car to move according to device tilt, use this

FrontLeftWheel.steerAngle = 20 * Input.acceleration.y; /* you can use any number instead of 20 for sensitivity. FrontLeftwheel is the wheel collider attached to front wheels. .y is for landscape orientation.
FrontRightWheel.steerAngle = 20 * Input.acceleration.y;