On a new install, all navigation links begin with 'Directory' and your front page title will be "Directory: Main". You're likely to want to customize this to something appropriate for your website. First go to Admin Panel -> Settings -> System Config and set your site title appropriately. That'll get used by default, and you can go to Admin -> Templates -> Settings and change the titles scheme to customize it to be different.

To edit the ending part of your titles and navigation bits for various pages, go to Admin Panel -> Customizations -> Language. There you can either click "page titles" or use the filter option at the top to search for the language item with the title text you're looking to replace. The start bit for the navigation will be in the language item named navorigin.

For custom pages not handled in the language, you can use Admin Panel -> Templates -> Page Titles to set titles. Be sure to supply the non-rewritten versions of the URLs.