First of all you should check out the documentation:
Menu HowTo would probably be a good place to start.
The basic process is this:
- Go to tiki-admin_menus.php
- Create a new menu, adding all the items you want (note the menu ID of the new menu)
- Go to tiki-admin.php?page=look (Look and Feel Admin)
- Under "General Layout Options" looks for "Site Menu Bar" and set the menu ID to the menu you just created.
The most difficult part is making sure that users only see menu items they are supposed to see. You need to understand permissions for this and also what the different "features" are.