Select a face of an object ingame

Hi!

I'm looking for a way to select individual faces/vertices in a mesh, based on my mouse position ingame, and then manipulate their coordinates. The idea in my project, is to increase or decrease the vertex/face distance in a given direction, from the world center.

Here's the code I have so far:

 function OnMouseOver(){
        if (Input.GetMouseButton(0)) {
            // need code to select nearest Vertex of my mouse here.
            //GameObject.Find("Sphere").??
         }
 }

I have done some searching, and similar questions have been asked in here, but they were not replied (and it is a long time ago), therefore I decided to ask again :)