Need user login credential same for both site

My WP site hosted with gooddy and would like to create multisite.

I have already created my main site on wp, now just wanted to understand how I should go forward.

For multisite, should I start with creating sub-domain and installing wp in this.

I want once any user/subscriber create an account either on main site or subdomain site, he can then use same login credential/eligible auto logged in for both the site.

Is there any plugin & coding required on this to create like site1.multi.domain.com and godaddy do i need wildcard dns setting done from godaddy.