To add a link to a menu you simply log in to your Joomla control panel and in the 'Menus' drop down menu select the menu you wish to add the link to and click the 'New' button.

NOTE: The 'Main Menu' is the horizontal menu just below the header.

Once you have clicked the 'New' Button you will see a list of options. There are really only two options that you are going to be using regularly and that's the 'Articles' link and the 'External' Link.

Link to an Article 

To add a link to an article click the 'Articles' link then click 'Article Layout'

You then need to give the link a title and choose the alias which is what the URL of the page will be. If you leave the alias field blank it will simply add the title as the alias automatically.

The access level allows you to make the article public, meaning any visitor to your site can view it or registered, meaning only registered members on your site can view it. Adding registered content is a good way to encourage people to register to your site.

Then in the 'Parameters - Basic' menu click select and click on the article you wish to link to.

Click save and you are done.

External Link 

External links are used to link to anything outside of Joomla. For example if you use Linkmachine or Blogger even though it resides on your site it is still outside of Joomla therefore considered an external link.

Click the 'External' link and follow the instructions above. However this time rather than selecting the article you simply enter the URL you wish to link to in the 'Link' field.