How to turn on an LED using a button

I recently just got a photon and I am trying to do something extremely simple but can’t seem to figure it out.

All I want to do is turn on an the board LED when there is current, and turn off the board LED when there is no current.

How can I do this?

To be clear, I know I can just add the LED into the circuit but that’s not want I want. I want other actions to take place when the LED turns on and off.

What have you tried so far?