1. Open the Menus and click on the default menu (the one with an asterisk).
  2. Find and click the default menu item (the one with the yellow star).
  3. Select the Parameter tab and enter your title in the Page Title box.

As you can imagine, for each page you can define a new Page Title by editing a menu item on that page. If the Page Title field, leave it blank, Joomla Page Title will announce the title of item.