If your blog is hosted here at WordPress.COM, you don’t need to do anything. Google will find you, but it may take a few weeks. If you feel that you must, you can give them a ping by going here:
You don’t need to ping them, as this website automatically does it whenever you post; and if you do ping them, just do it once. Note that they DON’T want http, just the actual blog name, such as MyBlog.wordpress.com
@elisabeth, tagging a loooot can mean that your blog is not showing up on the global tags pages, and search engines get suspicious if you use too many. With the global tags pages here, a combined total of 10 or 12 tags + categories will keep you on the global tags pages, and they bring in a lot of search engine juice.