Locking your cellphone is a good idea if you often misplace it, or if you have private information on it. Most phones will allow you to lock them manually, and also will lock themselves if you leave them alone for a certain amount of time.
Access the security settings of your cell phone. This is usually listed in the settings menu that is accessed through the main menu of your phone.
Select the "Phone Lock" option. If your phone does not lock automatically it is most likely set to "Off." Switch it to "On."
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Select the preferences for the lock setting. This will usually require you to select how much time you would like before your cell phone automatically locks itself. Your options are usually between 15 seconds and one minute.