Have you done stuff with other inputs? Using mouse buttons is no different from using any other button. Just set up your actions in the Input Map tab of Project Settings, and give them mouse button bindings.
When you want to start getting input in your script, (probably in your ready() function), you have to enable it with: set_process_input(true). Just like you have probably done with set_process(true), and the corresponding func process(delta):
func _input(event):
if event.is_action_pressed("go_right"):
do stuff . . .
elif event.is_action_pressed("go_left"):
do opposite stuff . . .
Or if you want to check it every frame, or whenever, you call Input.is_action_pressed("go_right"), which gives you a true or false value.
