How do I just submit one sitemap index to Google that covers topics, posts etc.?

If it exists, where can I find it? So far I've been using a few because I don't know which is the right one: /news/sitemap.xml /news/Sitemap.xml /news/sitemapindex.xml

Thanks,
Teeb

EDIT: for myurl.com/sitemapindex.xml i get a strange page that tells me there isn't enough information. What does this mean? Should I be using this sitemap instead of the one in the 'news' directory? My phpBB install is in the root of the domain btw.

I don't think it matters whether you have 1 sitemap or 10; Google figures it out. That said, I don't think sitemaps are very important as Googlebot is quite skilled at finding new content, especially if you have Google AdSense instrumentation. From Google's About Sitemaps:

Sitemaps are particularly helpful if:

• Your site has dynamic content.
• Your site has pages that aren't easily discovered by Googlebot during the crawl process—for example, pages featuring rich AJAX or images.
• Your site is new and has few links to it. (Googlebot crawls the web by following links from one page to another, so if your site isn't well linked, it may be hard for us to discover it.)
• Your site has a large archive of content pages that are not well linked to each other, or are not linked at all.

Google doesn't guarantee that we'll crawl or index all of your URLs.

phpBB-seo generates loads of bot-friendly links, so the need for sitemaps, in my opinion, is negligible.