Work in progress
When you create pages and put them in your primary menu, by default they will be sorted in alphabetical order. This is generally okay for a references or an index, but for your main menu, it rarely is the order you want to have.
To fix the order, go to your Menu Administration screen. The menu is under Administer » Site building » Menus » <name of the menu to edit>. Once you are on the menu page, you see the menu items preceded by a cross which you can use to move the item up and down by clicking on it and holding your mouse button down until it is at the right place.
This will change the Weight of the menu entry. Although you could change the weight by hand, using the drag and drop mechanism is definitively a lot easier!