When you connect the HDMI cable to your laptop, the other device will automatically become a second monitor. You will see your normal stuff on your laptop and a blank screen with a Taskbar at the bottom like below but with your background.
In order to use this as a second screen, you can simply click at the top of whatever App you wish to display and drag it across to the second screen:
Or you can right click on the desktop anywhere and select Display Settings:
Scroll Down to ‘Multiple displays’, usually the ‘Extend these displays’ option is selected.
Click the drop-down box and select ‘Duplicate these displays’
Then what is displayed on your laptop will be displayed on the second screen as well.