Most Google users are ‘searching’ on mobile devices, so it only seems fair that Google ranks websites accordingly. In the coming months, Google’s rankings will primarily focus on the mobile version of websites, only looking at desktop if there is no mobile version available.
If your website isn’t mobile friendly, Google has said this will not affect your rankings, as it will continue to crawl the desktop version if mobile is not available. Although, now would be a great time to join the mobile revolution. The reason Google is starting mobile-first indexing is due to the overwhelming number of mobile users which has continuously outgrown desktop since 2014. Making your website mobile-friendly is only going to be beneficial to you.
If your mobile site is not as impeccable as your desktop version, it would be a good idea to improve it now in preparation for the change. Websites with a responsive design will not need to worry, however websites with a desktop and mobile version with less content and links on mobile should look at trying to update as soon as possible.