How to revert function keys to original use

I'm still learning how to use my new MacBook Air, so function keys are very new to me, but at the moment I prefer the keys to be used for their main purpose (F1 and F2 for brightness, F10-F12 for volume, etc.). The problem I have is that my laptop has recently stopped this, and is trying to use the key for the special functions (the only one that is programmed is F11, and it shows the desktop), but I don't remember doing this to my laptop, or how to revert it.

I quickly browsed through the suggestions, but I couldn't get any results when changing the system preferences on my keyboard in settings. Maybe I'm not doing something right or forgot a step? I don't know what to do.