Use a ray, e.g.

var ray = Camera.main.ScreenPointToRay (Input.mousePosition);
var hit : RaycastHit;
if (Physics.Raycast (ray, hit, 100)) 
{
  Debug.DrawLine (ray.origin, hit.point);
}

Docs here.

Send a ray cast from mouse position then you can get all the information you need from the raycast hit

var ray = Camera.main.ScreenPointToRay (Input.mousePosition);
var hit : RaycastHit;
if (Physics.Raycast (ray, hit, 100)) {
    var hitPoint = hit.point;
}

doc

var clickPoint : Vector3;

function Update()

{

var ray = Camera.main.ScreenPointToRay (Input.mousePosition);

var hit : RaycastHit;



if (Physics.Raycast (ray, hit, 100) && Input.GetButton("Fire1")) 

{

    Debug.DrawLine (ray.origin, hit.point,Color.red);

    clickPoint = hit.point;

}