Google and the other search engines will pull keywords from the content of your post. If you are talking about meta keywords, none of the major search engines pay any attention to them anymore since they have been so abused.
Categorize and tag your posts (maximum 12 tags per post), put keywords into your posts, but do not overdo it as search engines will lower your ranking.
WordPress.com has huge search engine ranking and all sites here benefit from that and they have done an exceptional job with making sure that all themes are optimized for the search engines. They automatically create your XML site map and keep it updated, and each time you post, the notify all the major search engines and services.