Creating a new menu item in Joomla is extremely easy. Joomla uses these menu items to create a menu of links as seen below.
These links typically lead to an article. Assuming you already have the article created, click on the menu you would like to add to from the "Menus" drop down.
One the next page, click the "New" button in the top right corner.
On the next page, you will select "Article", and then "Article Layout" as seen below.
On the next page, all you need to do is give the menu item a title and select the article you would like to link to in the right hand side of the page.
If you want to change the order of the menu items, go back to the list of all the menu items and click the little up and down arrows. You can also turn the menu items on or off. And that is all there is to it.