Menu Bar
Your main menu bar, (whether it be a horizontal menu template or a side menu template), is determined by all of your top level pages. These are the pages that are on the main level and not parented to any other page and can be easily identified by the yellow page icon next to them.
By default when you create a new webpage it will be a top level page, unless you parent it to another page or change its page type.
The title of your page will be what appears on the menu bar. So make sure not to have too long a title as these are also your menu buttons.