Use A Responsive Theme

As of April 21st, 2015 Google will rank mobile-friendly websites higher in their search results.
“Starting April 21, we will be expanding our use of mobile-friendliness as a ranking signal. This change will affect mobile searches in all languages worldwide and will have a significant impact in our search results.”

http://googlewebmastercentral.blogspot.com/2015/02/finding-more-mobile-friendly-search.html

You can test your mobile-friendliness here:
https://www.google.com/webmasters/tools/mobile-friendly

It is highly recommended that you use a mobile friendly WordPress theme on your site:
https://wordpress.org/themes/tags/fluid-layout+responsive-layout/