I'm playing a game called fishing planet and I need to use f11 to change the pitch but it's the volume so I can't use it
System Preferences > Keyboard > Shortcuts > Locate "F11" and uncheck it for Show Desktop (or whatever its set as)
Hit the Fn key at the bottom left along with F11. Alternatively, in System Preferences, you can set the F keys to be used as F keys and not as volume etc., so then you won't need to press the Fn key every time.
The function keys on a mac have certain functions like volume, Expose, etc. Instead, you need the Fn key to press. so Fn+F11. On a windows on the other hand, you would use the function key for the F keys functions.
You can do such thing on a mac too in System Preferences > Keyboard > Use F1, F2, etc. as standard function keys.
Doing the above will make F11 keys function like on windows. If you need to adjust volume, etc. with this option checked, you use the Fn key for that.
I find this definition to be helpful towards the question:
The F11 function key will quickly slide up all your windows, revealing your desktop, and letting you interact with your files and folders. By default on MacBook / Pro / Air, and on the small keyboard for iMac, the F11 key is mapped to special functionality, like decreasing the volume. All you need to do is hold down the "fn" function key (bottom left corner of your keyboard) while pressing the F11 key to trigger the show-desktop command.