On the top right of the Nintendo Switch there is a cover for the game slot. Open it . If there is a game inside it already, press down on it to eject it and remove it from the slot. Next slide the game cartridge in of the game you wish to play until you here a click. The game should pop up as the first playable game in the Nintendo Switch's menu. Select the game by using the joystick then press A and the game will start.
Cartridge Issues
JoyCon Issues
One of the most common problems is the JoyCons won't detach from the switch. Push on the circular button on the back of the switch and push the controller back into place. Let go of the circular button and jiggle the controller a bit. Then press on the circular button and try to disconnect the controller again.
Another common problem is putting the attachments to the JoyCons on the wrong way. This will make the attachments hang over on one side.