Build something that works
Managing your website's SEO is a deep topic - there's a lot to learn and honestly, it's not my main area of expertise. Here are the basics though: to submit your website to Google, you'll need to go here (http://www.google.com/submityourcontent/website-owner/) and either add URLs individually (best if you just have a landing page or a few static pages) or submit a sitemap (recommended if you have many URLs to add).
Most Rails apps are more complicated than static sites though, and often there are new URLs being created all the time (e.g. any site with a user-generated forum). To deal with this, you should look into gems like sitemap_generator (https://github.com/kjvarga/sitemap_generator). It takes some time to set up, but can save you a lot of time in the long run.
To learn more about SEO-related markup, I recommend you take a look at resources like these:
It can take a long time to go through your content and mark up everything, but it will help Google's bots better understand the content your site offers. After submitting everything to Google, you should see your site appear in Google Search within a few days and hopefully rise in the rankings after that!