Factory Reset an iPhone is very simple, you do not need iTunes or access, long as you know the security code if you have. To do this, you must access the folder 'Settings' on your iPhone. If the phone is locked, this procedure will not help to unlock.
Once in 'Settings', locate the 'General' tab and press on it to format your iPhone. Remember that this option will remove all that content that is not factory, so that if the phone is yours, do not forget to make a backup before.
In 'General', down to the bottom of the page and tap on 'Reset' and 'Reset'. As with all applications, pictures, etc., iOS will also be the one with the mobile factory will be deleted. So if iOS7 and now offers on the iPhone you want to reset is above 5, you must reinstall it if you want to continue having it. Cydia So will disappear and you will have to jailbreak again.
Within 'Reset', select the second option, 'Erase All Content and Settings'. If you have activated the security code, you must submit it to completely reformat your iPhone.
Now it will ask you if you really want to delete all content from your iPhone and leave it as it just left the factory. If so, tap on 'Delete iPhone'. The restore process can take several minutes.