vehicle moveing on the terrain

I just looked for a simple vehcle script that can make a vehicle move around the terrain and being able to climb montians (not spoting and avoiding obsticle on the way). All I found is the same problem but when the vehcle is controlled buy the player, and not when the vehcle is controlled only buy the computer.

So my problem is this: I made a simple vehicle which all it do is following me. I made a simple script so it will be able to find me after spawning, rotate and move forward. When I using this script the vehicle is flying to my direction and not stick to the ground like it should, so I added a rigidbody to it and now it falling trough the ground, even if I adding a mesh collider to it.

How I making the vehicle stick to the groud when it folloing me and not fly or falling trough the ground?

This is my script:

 var speed : double = 50;
 var damping = 3;
 var target;
 var targetDistance : double;
 var attackRange = 60;
 
 function Start ()
 {
    target = GameObject.FindWithTag("Player").transform;
 }
 
 function Update ()
 {
    targetDistance = Vector3.Distance(target.position, transform.position);
    
    //movement
    if (targetDistance > attackRange)
    {
       print ("is active");
       var rotation = Quaternion.LookRotation(target.position - transform.position);
       transform.rotation = Quaternion.Slerp(transform.rotation, rotation, Time.deltaTime * damping);
       
       transform.Translate(Vector3.forward * speed * Time.deltaTime);
    }
 }

Sorry for poor english.