How to Cast from my PC to my Apple TV

On a PC the only official method is to use airplay to stream content from within iTunes. Outside of that you will need to rely on 3rd party options such as AirParrot.

Also try Plex. Download the server app to your PC (for free), and then the Plex app to your apple tv. You can then stream from your PC to your devices anywhere in the world, and not have to be on the same network.