Check the documentation for InputEventMouseButton
For a simple answer you could create a new
_input handler like:
if event.type == InputEvent.MOUSE_BUTTON and event.button_index == BUTTON_LEFT and event.pressed:
It's also worth learning how to use InputMap to properly handle inputs from different sources. I think all of this is quite well covered in the docs, do make sure to follow the tutorials there, learning how to search and navigate the documentation will help you learn the engine much more quickly :) Good luck!