To put the iPhone into DFU mode so you can do an iTunes firmware restore follow these steps:
Open iTunes and connect the iPhone to your Mac.
Press and hold the Home button and the Sleep/Wake button at the same time.
After exactly 10 seconds release the Sleep/Wake button. Continue holding the home button until you iTunes pops up a message telling you that it has detected an iPhone in recovery mode.
The iPhone screen will remain black like this:
NOTE***: It may take a few attempts to get your iPhone into DFU mode. Generally, I hold down both buttons then release the Home button just before I think the Apple logo would appear. If you are still holding both buttons down and you see the Apple logo you are holding them down for too long!