Hey Hanah Steven,
There are many misconceptions regarding the ranking in this regard.
"Google recommends Responsive web design but does not prefer it over how it is accomplished" (from the video suggested below). Until and unless your website is mobile friendly there is no need to worry about anything. Just make it responsive, both mobile first / desktop first are same w.r.t to the ranking until and unless the website is responsive.
Make sure you get the rest of the SEO things right too in order to get the right results.
You can know more about it here. This talk was given in GDD India (Bangalore) last year. I suggest you to watch it completely. It will clear most of the misconceptions and give some insights.
As per Milica's suggestions those tools will surely help you validate things like speed, block resource loading, overall performance etc.
Hope this helps.
