Simple guide to put your iPhone in DFU Mode. This mode is necessary to restore your iDevice when it is having major software issues.
Step 1) Open iTunes and connect your iPhone.
Step 2) Press and Hold the Home button and the Sleep/Wake button.
Step 3) After around 10 seconds, release the Sleep/Wake button but continue to hold the Home button.
Step 4) Your iPhone should have a black screen or a screen with the connect to iTunes graphic. iTunes should pop up and say that a device was found in Recovery Mode. This is good! Now restore your iPhone like normal using this guide here!
Note: If you are holding the Home button and Sleep/Wake button too long, you will know this because your iPhone will restart and you will see the Apple Logo. Just keep trying, sometimes it takes several attempts to get your iPhone in DFU mode.