You can manually use Google Webmaster’s Tools sitemap submitter every time you update your blog. Or you can install WordPress SEO Plugin by Yoast and it will “ping” google automatically whenever you update your blog. You can check that it is working by visiting your webmaster’s tools interface (you are signed up to use it, right?) and check the sitemap submission page right after updating/adding a post. It should show Submitted = “Pending” and Processed = “<today’s date>”. I’m not sure if you have to manually submit the sitemap initially. Try it without doing so and see.