How to use onmouseover with inputdown?

Hey guys, I've been experimenting with a script that shoots rigidbodies, but I want it only to shoot when a plane is clicked on. I have a collider set up, and copied and pasted a script. However, when ever I try experimenting with the "onmouseover" sort of deal, I always get an error. (I'm the biggest noob ever). Can somebody tell me how to use a script that pretty much says that if the mouse is over the collider, and buttondown("fire1") it plays the normal script? Here's the script: var projectile : Rigidbody; var speed = 20;

 function Update () {
     // Put this in your update function
     if (Input.GetButtonDown("Fire1")) {
  
     // Instantiate the projectile at the position and rotation of this transform
     var clone : Rigidbody;
     clone = Instantiate(projectile, transform.position, transform.rotation);
  
     // Give the cloned object an initial velocity along the current
     // object's Z axis
     clone.velocity = transform.TransformDirection (Vector3.forward * speed);
     }
 }

thanks Also, I'm using the newest version of Unity.