Right click on desktop and select screen resolution, look where it says multiple displays if it show extended these displays the main desktop will be extended to the second screen (laptop main screen TV second screen) this is where you can open a document files on the main screen and drag it across to the second screen (you usually drag it to the right it will disappear of the main screen to the extended screen).
If you have it set to Duplicate this display what ever you open on one screen will appear on the second.
You can if you want make the TV the main screen or display just the TV but would recommend changing it back before you turn the laptop off.
With HDMI you should also be able to get the sound via the TV. Right click on the speaker icon bottom right of the taskbar, select playback devices and make the HDMI the default remember to change this back to speakers when the HDMI is disconnected.