First and foremost, this change won't apply to everyone. Google will send you a notification via Search Console to let you know if you site is being indexed with mobile first. It's important to note that this doesn't mean there are now two indexes, it's still one index. It's just a change in how Google will review your content - as a mobile user. Mobile Indexing also doesn't affect what factors (signals) Google uses to rank your content, but it will place additional importance on the quality of your mobile website and how it performs on a mobile device.
So how are you impacted by this and what should you do about it? The short answer is: most sites won't be affected by it, and that's because most sites have a desktop and mobile version on a same URL - using responsive/adaptive web design techniques. Sites that will be affected are those that use separate URL's for content on desktop and mobile and those that serve content dynamically depending on your device type - a differentiated content strategy. For these sites, your index/sitemap is varied between mobile and desktop users and as such this change to Google's approach will require some careful planning.
To be on the safe side, we recommend reviewing your website on a mobile device. If it doesn't work, or its not legible, or there are usability issues then these need to be fixed ASAP. Also, check that the content and structure of the site retains a similar consistency with your desktop/full-feature site - to ensure that your message is relayed across all devices. Finally ensure that the URLs you use for mobile are consistent with the standard URLs for the remainder of the site.
With consumers increasingly engaging with businesses through mobile devices, the emphasis on being mobile-enabled is increasingly important. Google's mandate in that direction is simply confirmation of the growing trend across users.
If you're unsure whether or not your website is mobile ready or if this change is going to affect you and you'd like to discuss your options, let's have a chat about what you can do to ensure the best possible performance of your site.