In game FUNCTION KEYS!!!!?

Hi I have a Dell Studio 17 and I play games that require use of the function keys (f1-f12) so when i attempt to use them in game they just do their regular factory function for instance when I press f4 my brightness lowers on my computer screen in game so how to I make it to where it wont do their function? Yes I do have a Fn key by my Ctrl but it doesnt seem to work.