At first, to play Pokemon GO on PC you should download android emulator named "Nox" and install there Pokemon GO APK.
http://en.bignox.com/pokemongo - there you can download Nox and APK file of the Pokemon GO
When you got both this files, install Nox and Run it. To install Pokemon GO just drag and drop Apk files on the Nox window.
Now you are ready to catch polemons from your pc!