pause .... .

function Update ()
{
if(Input.GetButtonDown("Fire1"))
   {
   Time.timeScale = 0;
   }
}

this should pause the game when the left mouse button is pressed.

OrangeLightning's answer is the easiest way.
If you want a more complex solution with menues look at the wiki: PauseMenu

This only works for gameobjects whos actions are based on time and not framerate though. You should also add an audiopause and add a if(Time.timescale == 1.0) to all non-time base objects.

I accidentally find this video. https://www.youtube.com/watch?v=KPaEnLpu57s