oh that's easy. Use the Mouse Event OnClicked(Whatever Object). This event will fire when your object has been clicked by the mouse. In there you should probably store the UID of the last object selected. Then your other controls can use that to take action.
Also put all your selectable objects into a family.
I'm not on a computer with C2 right now but this is basically how you do it...
Make a family of selectable objects called Pieces
LastSelectedObjectUID = Pieces.UID
System Pick Object by UID(LastSelectedObjectUID)
Rotate Pieces x degrees clockwise