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.”

You can test your mobile-friendliness here:

It is highly recommended that you use a mobile friendly WordPress theme on your site: