Google Mobile First Indexing upgrade: what you have to know

In order to help webmasters prepare for the upcoming change (which has currently been rolled out in a few select locations), they have provided some guidelines to follow:
If your Mobile and Responsive website are the same

If your current responsive website, or your dynamic website currently has identical content and markup for both your mobile and desktop version, you should not need to change anything.
If your Mobile and Responsive website are not the same

If you have a website where the primary content and markup is different across mobile and desktop, then you will need to make changes to your website:
Structured markup

Google recommends that you should:

Implement structured markup on both your desktop and mobile version
You should test both versions of your website (where you have a separate mobile version) in the Structured Data Testing Tool, and compare the output.
    When adding structured data to a mobile website, avoid adding large amounts of markup that isn’t relevant to the specific information content of each document tool to verify that your mobile version is accessible to Googlebot.
With the new Mobile First Indexing, remember to make sure all relevant content shows on your mobile version, including structured data.
  
Canonical Links
In the announcement post, Google confirmed that:

Websites do not have to make changes to their canonical links; we’ll continue to use these links as guides to serve the appropriate results to a user searching on desktop or mobile.

Site Verification

For those of you who have a different portable form on your site, and that site is setup all alone subdomain, for example, m.yourdomain.com then you should ensure that subdomain is confirmed in the Google Search Console.

Here is the thing that Google said:

On the off chance that you are a site proprietor who has just confirmed their desktop site in Search Console, please include and check your portable form.

No Mobile or responsive site

On the off chance that you are one of only a handful couple of sites that don't at present have a portable variant of your site, and have not actualized responsive styling then don't stress. Google will in any case record your site, however they will see utilizing their portable crawler.

In the event that you just have a desktop site, we'll keep on indexing your desktop site fine and dandy, regardless of the possibility that we're utilizing a versatile client specialist to see your site.

Clearly, with the Mobile-accommodating positioning support, and for client fulfillment we would prescribe executing a versatile rendition of your site immediately.

Try not to dispatch versatile site before it's prepared

Google gave a notice against racing to reveal the versatile form of your site before it is completely entire, generally your site will be founded on that broken rendition.