How do you lock the screen? I am a new user to an iPhone

Hopefully you have setup a passcode to unlock the screen. If you have then great and if you have not then there is no reason to even lock the screen. PRESS once on the RIGHT SIDE BUTTON (Biggest button) of your iPhone 12. That will lock the screen.

Axel F.

Turning the screen lock on or off in your iPhone

1. Tap Settings.
2. Tap General.
3. Tap Auto-Lock.
   1. To turn on automatic screen lock: Tap the required period.
   2. To turn off automatic screen lock: Tap Never.

You can also adjust the time to autolock the screen in: Settings>Display&Brightnes>Autolock...

Regards

Giulio