var hit: RaycastHit;
var ray;
private var target : Transform;

function Update () 
{
    if(Input.GetMouseButtonDown(0))    // for standalone
       {
         if (Physics.Raycast (ray, hit, 100)) 
           {
              if(hit.collider.gameObject.tag == "TagName")
                {
                 target = hit.collider.gameObject;
              }
       }
}

I would use the unity function OnMouseDown and then change a select variable you can have in a master GUI script.

def OnMouseDown():
    currentPlayersGameObject = GameObject.Find(_gameState.WhosTurn)
    currentPlayer = currentPlayersGameObject.GetComponent[of PlayerScript]()
    if currentPlayer.IsPlayerAI == false:
       currentPlayerGUI = currentPlayersGameObject.GetComponent[of GuiScript]()
       currentPlayerGUI.SetSelected(gameObject)

Search for mouse raycasting, you'll find what you need. The idea is to convert the 2d cursor position in 3d, raycast, and then hit.transform gives you the transform of the object you are pointing. Plenty of examples already out there!