'F' as in the F1-F12 keys? If you have the aluminum keyboard, simply hitting the 'fn' key and then the 'F' key you need to use will work. Alternatively, you can go to System Prefs-->Keyboard and Mouse. On the Keyboard tab, check the box that says 'Use all F1, F2, etc. keys as standard function keys', and you can bypass hitting the 'fn' key. The downside to this is that if you use Dashboard, Expose, or Spaces, you will have to hit the 'fn' key and then the corresponding 'F' key to use them.