Contact us forms are built into all Shopify themes. See this page for instructions on how to create one in your own store. If you want to use Google Shopping, you'll need to list two of the following three options: 

• Phone number
• Email address
• Postal address

These can be included on the same page as your contact form, or can appear in a different place on your site, such as the footer. 

Don't forget to include your contact page in your navigation.