If your charity's website has more than a few pages, you should consider creating a site map - a hierarchical list of the pages on your website. Here is the sitemap for the Acorn PR website.
Site maps help users navigate through your charity's website by allowing them to go directly to pages within subdirectories. Having a site map on your site also ensures that search engines can find and index all the pages on your site.
Once you have created a sitemap, link to it from every page on your charity's website, so users can reach it no matter where they land on your site.
It is also worth submitting a site map to Google, particularly if your charity's website is new and Google hasn't yet discovered it, or if some pages are hard to find. Google's Webmaster tools lets you upload a sitemap and track other information about your website.