An RSS feed element is automatically added to the sidebar of any blog you create. The RSS feed allows blog visitors to receive automatic updates about your blog if they are using a feed reader like feedly.com or theoldreader.com.
The link to the feed looks like this:
If you accidentally delete it or don't see it, you can add it back by dragging a Blog Feed element to the sidebar.
RSS feeds are only available for blogs, not regular pages. If you create more than one blog on your site, each blog will have its own feed.
You don't need to do anything at all to activate your RSS feed. It's simply there and will start working from the moment you publish your first blog post.