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The Google Pixel has initially limited third party development on its Daydream VR platform, meaning that you can't use the Daydream App to directly view 360 content stored on your phone created by popular 360 cameras like the Ricoh Theta S.  If you still want to use Daydream to see 360 content that you can create and store on your phone, the easiest App to use that is supported by Daydream is Cardboard Camera.  

With Cardboard Camera, you can use any camera's phone to take a panoramic shot by physically rotating your phone around 360 degrees.  You can then feel immersed in the photo using the Daydream View.  Unfortunately, you can't take 360 videos using Cardboard Camera.

To use Cardboard Camera, you need to download and install it on Google Play. 

  

  

  

If you want to actually to view 360 video content taken on your Ricoh Theta S, another option is to upload that video to watch on YouTube VR but it requires some technical effort to actually process the 360 video correctly.