If you have the requisite WiFi network, a compatible device that is connected to that network, and the desire to use any of the services that can stream through Chromecast, the next step is getting one and going through a few simple steps. You will need to know your WiFi password for setup.
It may also require some configuration of your WiFi router under certain circumstances, and it's possible for your router to be incompatible with the Chromecast. Google's support site has a Chromecast Router Compatibility page with a long list of compatible routers, which also includes the firmware version, possible workarounds for certain issues and contact information for the router manufacturers.
Barring connection issues, you simply plug the USB power cable into the Chromecast and into a power source, and plug the Chromecast into an HDMI port on your television set. The HDMI connector on the Chromecast can also be plugged into the included HDMI extender, which can be connected to the TV, either if there is no room for the device directly in the HDMI port or if you are having WiFi reception issues.
Suitable power sources are either a USB port on a device like your TV, or the included power adapter plugged into a wall outlet. There's a warning on the Chromecast site and in the documentation not to plug the USB cable into a port labeled "service," but only into one clearly marked as a USB port.
Once the device is connected and powered, switch the TV's input to the right HDMI port, and the television will display a "set me up" screen that prompts you to visit google.com/chromecast/setup. You should go to this site on the device you want to use to control the Chromecast. The setup screen also shows the default name of the Chromecast.
From there, you'll download, install and run a Chromecast app if you're using a smartphone or tablet (it will direct you to the appropriate app store), or the Google Cast extension for the Chrome browser if you are using a desktop or laptop. If you are using a computer that doesn't already have the Chrome browser, you will have to install that, as well.
If you go to the setup site from a device that runs an unsupported operating system, it will warn you but let you try to install software anyway. The Google Cast extension may install on some non-supported OSes, and in some cases may even work to an extent, but the functionality will likely be unpredictable.
In the case of the Chromecast app, it will take you through several more steps to set up your new device. You connect your phone's WiFi to the Chromecast itself rather than your home network. A code should appear both on the TV screen and on your launch device screen. You confirm that they match to move forward. You'll be prompted to change the name of the Chromecast to something personal. You then select your home WiFi network, enter your password and hit a button to finish set up.
Once it's connected to WiFi, the Chromecast will probably do an auto-update to grab the latest software, which may take a few minutes. Anytime updated software is available, the device downloads and installs it automatically, whether you're performing setup or not.
After all this, provided there are no connection issues, your mobile device or computer should be ready to stream to your TV via Chromecast. The next step is opening an app and casting away.