See the FPS tutorial for weapon aiming. FPS Tutorial

Essentially, you just cast a ray with Camera.ScreenPointToRay(); Here's a simple example.

// Draws a line in the scene view going through the mouse position.
// from the lower-left corner of the screen

function Update () {

    var hit : RaycastHit; //unnecessary in this example.

    var ray = Camera.main.ScreenPointToRay (Input.mousePosition); //ray from
    // through the mousePosition.

    if(Physics.Raycast(ray, hit, Mathf.Infinity)) { //does the ray collide with 
    // anything.

         //add stuff here for finding type of object and such.
         Debug.Log("Hit Something at mouse position");

    }

    Debug.DrawRay (ray.origin, ray.direction * 10, Color.yellow);
    //Display the ray.
}

//untested watch out for typos.