If you ever mess up your iPhone or iPod to the point that you need to restore it and start from scratch, you’ll need to put it into “Recovery Mode.” Start by powering it down — simply hold the home button for 5 seconds and move the slider to the right.
After your device shuts down, press and hold the home button and plug your iPhone (iPod) into your computer using the USB cable. That’s it. iTunes should recognize that your device is in recovery mode and prompt you to restore it from your backup.
This is particularly useful if you jailbreak your iPhone and things go wrong.