Are you trying to get the mouse coordinates?
get_local_mouse_position() are both methods of
CanvasItem, so if you're using a 2D node, you can use "CallSelf" and call one of those functions.
Otherwise, your best bet is to add mouse click action(s) to the Input Map, and use the
Input.is_action_pressed() function. You can get this from EngineSingleton -> CallInstance.