Player vehicle Movement using rigidbody

Hi guys,

Need some real help here for a school project and I am a novice with unity. I am aiming for player movement using rigid body that works similar to a street vehicle. I need physics that accelerates through to a top speed and once in motion I want the object to stay in motion until drag slowly stops it or brakes are applied. The code I have at the bottom moves like a boat in water, it doesn't have good turning and some weird rotation issues. plus no gauge for top speed. So I mainly narrowed it down to three questions for you guys.

1) What should I be using to move rigid body forward and backwards? forces or velocity?

2) What should I be using to turn rigid body while moving forward? forces, torque, or angular velocity?

3) How should I accelerate gradually to a top speed or velocity?

here is my code that I have been doing for about two weeks and tried every kind of combination that I know. So any help with this would really be appreciated.

     var tForce : float = 100;
     var mForce : float = 300;
     var drag : float = 1.5;
     
     var forward : boolean = false; 
     var backward : boolean = false;
     var right : boolean = false;
     var left : boolean = false;
     
     var r_angleVelocity : Vector3 = Vector3 (0,100,0);
     var l_angleVelocity : Vector3 = Vector3 (0,-100,0);
     
     var forwardVector : Vector3;
     var rightVector : Vector3;
     var leftVector : Vector3;
     
     var forwardLeft: Vector3;
     var forwardRight: Vector3;
 
 function Update () 
 
 {
 
     if(Input.GetKey("w"))
     {
         forward = true;
     }
     else
     {
         forward = false;
     }
         
     
     if(Input.GetKey("s"))
     {
         backward = true;
     }
     else
     {
         backward = false;
     }
     
     if(Input.GetKey("d"))
     {
        right = true;
     }
     else
     {
            right = false;
     }
     
     if(Input.GetKey("a"))
     {
        left = true;
     }
     else
     {
            left = false; 
     }
     
 }
 
 function FixedUpdate()
 
 {
 
     var r_deltaRotation : Quaternion = Quaternion.Euler(r_angleVelocity * Time.deltaTime);
      var l_deltaRotation : Quaternion = Quaternion.Euler(l_angleVelocity * Time.deltaTime);
      
      forwardVector = (Vector3.forward * mForce);
      rightVector = (Vector3.right * tForce).normalized;
      leftVector = (Vector3.left * tForce).normalized;
      
      forwardLeft = (forwardVector + leftVector).normalized;
      forwardRight = (forwardVector + rightVector).normalized;
      
      if(forward == true)
      {
          rigidbody.AddRelativeForce(forwardVector,ForceMode.Force);
          
      }
      else
      {
          rigidbody.drag = drag;
      }
      
      if( backward == true)
      {
          rigidbody.AddRelativeForce(Vector3.back * mForce, ForceMode.Force);
      }
      
      
      
      if( right == true)
      {
          
          rigidbody.MoveRotation(rigidbody.rotation * r_deltaRotation);
 
          rigidbody.AddForce(forwardRight, ForceMode.Acceleration);
          
     }
      
      
      if( left == true)
      {
          
          rigidbody.MoveRotation(rigidbody.rotation * l_deltaRotation);
 
          rigidbody.AddForce(forwardLeft, ForceMode.Acceleration);
      }
      
  }