I have a iphone X10. Could somebody tell me how to lock my screen

Pressing the side button locks the screen.

You can back up your phone by going to Settings/<your name>/iCloud, then tap Backup, and Backup Now. Or if you turn on Automatic Backups and leave your phone plugged in and connected to Wi-Fi overnight it will automatically back up every night.

You can also back it up to your computer with iTunes.