On your external monitor's settings, just make sure Display is selected, then change Rotation to whatever you need it to be.
Alternatively, you can enable Mirror Displays just check the box next to it to have your external display show the exact same thing that appears on your Mac's screen. When video mirroring, your Mac will try to use the same resolution, or closest resolution, that it uses on its built-in display to the external monitor.
If you don't want an extended desktop, and don't want to mirror your displays, then you probably just want a bigger screen for your computer. This can be done using clamshell mode. In order to do this, your Mac notebook will need to be connected to its power adapter, and you'll need an external keyboard and mouse or trackpad.
If using a wired keyboard and mouse, make sure they're connected, then simply close the lid on your Mac notebook. If you're running OS X Lion or later, your desktop should automatically appear on the monitor after flashing blue for a second. For those of you with Snow Leopard or older, you'll have to press a key or click a mouse button to see the desktop.
To exit clamshell mode, put your computer to sleep, then disconnect the external monitor from your Mac's display port. In your external monitor's settings, depending on the type of monitor you have, you may have other options you can customize, under the Options tab.
To make any changes, you'll need to click on the lock icon and enter your admin password. Then, depending on your monitor, you can enable its power button to put the computer to sleep or wake it, or do nothing at all. You can also disable the brightness button on your monitor.
Sign Up. It would be great to be able to watch netflix on the tv while I do some work and search the internet on my actual computer Yes it does.
You can cast a single tab straight from chrome and it works fairly well. You can also cast your entire screen but it's in beta and doesn't cast very well.
I think OP wanted to know if the Chromecast can be used as a second monitor, which afaik it can't. Its a limited "second monitor", only being able to cast tabs or supported apps. It can mirror the desktop, like you said, but no extending the screen. It's not a second monitor in the traditional sense.
You can, however, stream netflix or other to the cast. If you'd want to watch a movie while working on your computer, cast Netflix or the Chrome tab hosting Netflix to the Chromecast and watch it there.
You can cast tabs to the Chromecast: this is more like mirroring your display than having a second monitor though you can minimize Chrome once the tab is casting and it will not stop the cast.