STEP 1. First you have to power off your iPhone by simply holding the button on the top of your iPhone and then slide the bar over to power the phone off.
STEP 2. Now you have to hold down the home button.
STEP 3. Now you have to connect your iPhone with the computer using usb sync cable at the same time iTunes must be running.
STEP 4. After some time you will be able to see the iTunes logo over the sync cable on the screen.
STEP 5. Now you will be able to see a pop up window which will verify that the iPhone is in recovery mode.
STEP 6. On your computer screen you will see a pop up window which will say that you must restore this iPhone before it can be used with iTunes, simply click on OK button.
STEP 7. Now you have to click on the restore button which is at the bottom of the screen.
That’s it. In this way you can restore you iPhone. If you found any difficulty in following the above steps then feel free to ask in the comment section below.