To turn on the caps lock on the iPhone you need to double tap the shift key. To turn off the caps lock once it is on, tap the shift key once. If you double tap the shift key and you can't gets the caps lock to turn on, then you need to set your preferences to enable the caps lock. To do this you go to Settings > General > Keyboard and make sure that Enable Caps Lock is set to on. Once it is, the double tap of the shift key will enable it.