How do I get my phone to play on sonos

Press the button/link that shows the speaker options, depending on what music source you're using, and it will be listed there.

For example, when playing a song in iTunes, click on the song banner at the bottom of the screen so it fills the phone's screen. At the bottom you'll see a little round "target" icon with a triangle at the bottom. Press that and you'll see the available speakers, including your Sonos.