How do I lock my Blackberry Curve 8520?

On your homescreen there should be a padlock at the bottom (if you have the 4.5 OS that is) but if you do this, you may not be able to unlock your phone as you have to press the A button to unlock it if I remember correctly? Or you can back up your files and restore it. If that fails then you should probably consider taking it to a phone shop :-)