how to restore my iphone 5

Try a Reset...

Reset ( No Data will be Lost )

Press and hold the Sleep/Wake button and the Home button at the same time for at least ten seconds, until the Apple logo appears. Release the Buttons.

http://support.apple.com/kb/ht1430

If no joy...

Connect to iTunes on the computer you usually Sync with and Restore

http://support.apple.com/kb/HT1414

Make sure you have the Latest Version of iTunes (v11) Installed on your computer

iTunes free download from www.itunes.com/download

Try resetting:

Hold both the home and power buttons at the same time until the Apple boot logo appears. No data will be lost.