Hello,
I think I can help you.
The correct name for the “main header” is the “Navigation” menu or just called “Menu”. Pages are added automatically to the navigational menu & you will have not control UNLESS you use a template / theme which allows you to use custom menus – then you can control which items are added… (….most newer themes now allow you to create at least one custom nav. menu. The default theme that comes with WordPress, “TwentyTen” allows one custom menu, I believe. I prefer to use themes which allows 2…. but that’s just my personal preference).
Now, go to the back of your site & look down the left side menu and look down the list – find the “Appearance” tab & click on it – then below it is a another tab / sub-menu called “Menus” – click on that, and see if your theme allows custom nav. menus, and if so, how many.
If it DOES allow custom menus – first you need to create & then name the custom menu(s). I usually name my menus “Main Menu” & “Top Menu” (or “Footer Menu” if the theme’s 2nd menu is in the footer). After adding & naming your new menu, you need to save it. You will also need to add it to the theme or it won’t display – look on the top left area of the menus page & there is a box with 1 or more drop-downs, with the name of your new custom menus you just created in it – if the theme allows 2, then you need to lock them both in (eg: custom menu #1 = “Top Menu”, custom menu #2 = “Main Menu”. Once you press “Save”, they are locked in to your theme & will now show up on the front-end of the site.
Next you need to add all the items (pages, categories etc) to the new custom menu(s). To do this you just tick the boxes in the “Add Pages” & “Add Categories” lists, & press add & they will then appear on the right of your screen, underneath the new menu (You can make some of them sub-menus by dragging / moving them underneath each other). When you are happy with your new menu structure, press “Save” again – check the front of the site & make sure you are happy with it, & that’s it!!
If your theme does NOT allow custom nav. menus – I suggest you maybe consider looking for a theme which does. There are ways of working around it, but I think it might be too difficult for you at this time, however if you really need to keep the current theme you could look at adding a menu to it (via code added directly into the template, or maybe a plugin etc….).
Hope I’ve explained it clearly enough for you to understand, but it’s easy once you get the hang of it – good luck!