In addition to the areas of your theme where you can install widgets, there is one further area that you can customise and that it the menu (or menus if the theme allows for more than one). The option for defining the content of menus is directly under the option for setting up widgets.
Most themes have one menu bar defined directly under the page header. This is referred to as the Primary Menu. If your theme provides spots to install additional menus then there will be additional fields in the Theme Locations area at the top left of the menus page.
To the right of the Theme Locations area is the area where your menus are defined. There will be a row of tabs across the top with one tab for each defined menu and one additional one with a + in it for adding additional menus. Just because your theme only provides for one menu doesn't prevent you from defining as many menus as you like. The extra menus can either be used as replacement menus if you want to be able to swap the menu around to change what is displayed or you can install a custom menu widget into any of the widget areas to display any extra menus where ever you want.
To create an additional menu click on the + tab, type in a name for the new menu and press the "Create Menu" button. This will create a new tab where you can define the entries that you want to appear in this menu.
The entries in the menus are defined using the three additional areas on the left of the page - Custom Links, Pages, and Categories. Entering values into one of these three areas and pressing the corresponding "Add to Menu" button will add a new entry to the end of the menu open on the right of the page.
The down arrow on the right of each menu entry will open the options for that particular menu entry allowing you to change the label used for the particular entry in the menu as well as the title attribute (what you want to display as a popup or in the status bar when the mouse is hovered over that entry). There are also options there to cancel your change or to remove the entry from the menu.
Switching the order of the entries in the menu is simply a matter of moving the mouse over the entry you want to move, pressing the mouse button down and dragging the menu option to where you want it before releasing the mouse button.
Once you have made all the changes that you want to make to the menu then you should press the Save Menu button to save your changes and (if the menu is currently displayed on your site) have those changes appear for people to see them.
Some themes have support for multi level menus where the main menu entries simply open a list of further menu entries. Even if the theme you are using doesn't have this support it is still possible to set up multi level menus for your site. Where the theme doesn't have the functionality built in we will need to install a plug in to add that functionality. Multi-level menus are usually called suckerfish menus as the person who created the first example of how to define such a menu used the names of different types of suckerfish as the names of the dummy entries in their example menu.