Mobile version of website need help for seo please

Hi,

I have created a mobile version of a desktop website.
So currently, my website have two version of page of every page.

I have one index.html whichi is desktop site www.mydomain.com

and I have created another index.html which is placed under a folder mobile/index.html

When someone open www.mydomain.com from mobile or tablet. It takes them to the url www.mydomain.com/mobile/index.html

Now my question is, which one should I index in google search? Should I let google crawl both version of my site, if I do this. There will be duplicate contents for index.html and other pages.

Also should I make mobile.mydomain.com instead of www.mydomain.com/mobile/index.html as URL?

Kindly please help.

Just choice e responsive theme, its will auto mobile version