If you're using MacBook keyboard, you will see a Fn key. When this key is pressed, F1-F7 keys will become standard function keys.

Fortunately, you can change this behavior so that without pressing down Fn key, F1-F7 keys are already functioning as standard function keys.

The good thing here is, you can easily perform keystrokes that include these keys, such as Control F1.

Here are the steps to change these function keys' behavior:

1. Open Apple ▸ System Preferences..
2. Go to Keyboard & Mouse
3. Click on Keyboard section
4. Tick the last checkbox labelled Use all F1, F2, etc. keys as standard function keys

The bad thing is, usually these keys come with Brightness and Volume Adjustment, i.e. after the changes you will need to press Fn to adjust brightness and volume.