You need to expand your question a little. Do you mean you want a blank screen, or to put the Computer into sleep mode. To "lock" it in sleep mode, would mean to need a password on wakeup. Are you signed in with a password?
If so, one way is to open the Control Panel (icon view). Open "Power options".
Select a Power Plan ("Balanced" is good)., and click "Change Plan settings"
In Put the computer to sleep", select 10 minutes. Then select, at the bottom, "Change advanced power settings" "Save changes"
In the first option "require a password on wake up", select yes and then "OK"
This will blank your screen and also put the computer into sleep mode. When you wake it up, by mouse shaking or whatever, it willl require a password to put you back into the desktop.
Another way could be to right clcik the desktop and select "personalise"
Select, over bottom right, "Screen saver"
Select a screen saver from the options. In your case, "Blank" might be a good idea. In the bottom square, select 10 minutes before resuming. "Apply"
Again, in both cases, make sure you have, and rememebr, your password.
Thanks for the reply and your explanations on how to "lock" the computer in sleep mode or to use a screen saver. I want to put the computer into sleep mode. When I reboot my PC and it powers up I need to press CTRL+ALT+DEL and I need to enter a password. If I lock" my PC in sleep mode, would I use the same password?