Here's a quick primer in RSS:
RSS stands for Rich Site Summary, which is simply a fancy way of saying it is a format used to deliver content from a source in a neatly formatted manner. There are multiple types of feeds available, and RSS is generally used as a catch-all for the different types f feeds. Most of the time the RSS feed for a site is very similar to the regular URL (for example, my blog is actually hosted at http://www.diabolicalminds.com/weblog/
and my original RSS feed was located at http://www.diabolicalminds.com/weblog/?feed=rss2
Another thing you could do is to use a third-party service to provide extra features to your RSS feed. If nothing else you can go to feedburner.google.com and enter your URL into the "Burn a feed" box and click next. That will provide you with the URL of your feed if it is already available, even if you choose not to continue the setup and use Feedburner.
As for setting things up on your site, I've never used the wordpress.com hosted service as I use a self-hosted Wordpress install, but I know the ability for you to setup the feeds is there (if not enabled by default). Hopefully someone who is hosted on wordpress.com can provide a little more insight into the specific setup for you.