since i am computer illiterate, how do i "lock my screen" to do an icloud backup...

Press the power button to put it in sleep mode (also known as locking your screen). Don't hold down the power button, just a quick press.

For future reference, you can manually back up to iCloud by going to Settings > iCloud > Storage & Backup > Back Up Now.

means leave your phone connected to charger and don't use it. after set up interval, like two or five minutes screen will go dark and it means it locked itself.