I'm trying to figure out if I need a subdomain (something like http://m.monicatrodriguez.com) to allow the site to be read on mobile devices, or something else, or what. Pretty basic, but I'm just learning this stuff.
No, you won't need to create a subdomain for your domain to be viewed on a mobile device. You may though want to ask your hosting provider if they provide something like Mobile View to help with formatting issues.
...Have two domains, one with .com the other with .mobi for Screen device & Mobile device respectively
...Have different web pages for each. -------If the websites are 'static HTML', just get two different web hostings, one for each. -------If the websites require database support such as SQL, then put all files on one web hosting, -------------use the .com as the main one for that webhosting -------------Use domain forwarding to forward the .mobi website to a file in that webhosting such as 'indexM.html'
In the shared idea, you would use the same .inc files on both sites where you need to search for things in the database, but have two different .php files, one for each.