Plugins at wordpress.com

Hi, sorry for, what I hope, is a relatively easy thing to resolve but I have just started building websites, and I’m learning all the time.
Basically, I want to install a plugin that will create a sitemap that google will then be able to find on my site. I’ve looked all over and I can’t find a plugins tab anywhere, not even on the admin page, which is where it seems to be for everyone else. Please Help!
Thanks
Joe Shaw