We do not manually assign keywords to sites, nor can webmasters submit a list of preferred keywords for which their sites will appear.
The best way to ensure that your site returns for your preferred keywords is to include them on your pages. Our crawler analyzes the content of webpages in our index to determine the search queries for which they're most relevant. If you create an information-rich site that clearly and accurately describes your topic, it'll likely return for your desired keywords.
Keep in mind that your site may be returning for your preferred keywords but may not appear in the first few pages of results. To determine if this is the case, search for your site using our site operator along with a keyword, like this: [ site:Google.com doodle ]. This search returns the following results: http://www.google.com/search?q=site:google.com+doodle
In general, webmasters can improve the rank of their sites by increasing the number of high-quality sites that link to their pages. You can learn more about how Google ranks pages at http://www.google.com/corporate/tech.html. Also, we always recommend reviewing our guidelines for creating and maintaining a Google-friendly website.