The problem with a device like the PiFace is that it has several drivers needed for it to work. My previous suggestion of trying to get a RTC working was based on experience. That was what I used to learn how to add a device to the FDT. Before that I had already done the flashing LED's.
They are where you need to start. Figure out where the pins correspond to your GPIO control software.
Turning on a LED can be even simpler if you use one of the onboard LED's. On RPi you have green LED and the red LED onboard RPI to manipulate.
They show up as ordinary GPIO pins and can be controlled with
gpioctl
.
I would call that GPIO 101. The most basic function from the GPIO pins without buying anything.
You literally can't screw up. Worst case you set the wrong pin on or off and something on the board freezes. Simply reboot and back to normal.
Some of this post is old but Vadim's site is quite useful and I have learned much from his posts.
https://vzaigrin.wordpress.com/2014/12/20/gpioled-in-the-freebsd-on-the-raspberry-pi/
The other useful site is Gonzo's website who has several examples of GPIO work.
https://kernelnomicon.org/?p=757