Press the Power button. If you've set up a screen lock, you need to unlock it.
Your screen automatically turns on when you have an incoming call.
To turn the screen off:
Press the Power button. When you set down your phone, it's a good habit to do this. You prevent accidental touches, save a bit of battery and keep it secure with the screen lock.
To keep your screen off and phone quiet when you don't want interruptions, such as at night, use Do not disturb settings.