Factory Reset an iPhone is very simple, you do not need or access iTunes, provided they meet the security code if you have. To do this, you must access the folder 'Settings' from your iPhone. If the mobile is locked, this procedure will not help you unlock it.
Once inside 'Settings', find 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 press on 'Reset' or 'Reset'. Just as all applications, images, etc., iOS will also be the one with the mobile factory will be deleted. So if now offers on iOS7 and iPhone you want to reset is earlier than 5, you must reinstall 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 just left the factory. If so, press on 'Delete iPhone'. The restoration process may take several minutes.