Is there a way to make my virgin mobile rumor touch lock itself every time it goes to sleep?

i know how to lock the phone, but i cant find a setting that locks the phone automatically.. thanks.Is there a way to make my virgin mobile rumor touch lock itself every time it goes to sleep?
i have the same phone and it should do it by itself..there isnt a setting for it...or do you mean password lock...if you mean password lock just go to settings and it should say LOCK PHONE and you can set your own pass code so when it dims out you have to set it again