Have you installed the app on your pc? Assuming so (or, firstly do so), choose the room and browse to find a source you want to play. 

If you mean play all audio wirelessly from your PC to Sonos then there is no way to do that. You can do it via an audio cable if you have a Sonos speaker with a line in.

If you mean how to play music files stored on your computer then this article explains how. 

https://support.sonos.com/s/article/257?language=en_US