Joomla is a php based open-source CMS toolkit. More you can find out from this document

Just follow the steps:

1.Create an article
2. Type some intro text, we need to put in the read more divider.
3.Click the Read More button at the bottom of the text editor.
4.Once you have the read more separation line, add some text beneath it.
5.Configuring the Article’s Parameters

Now that we have an article with intro text and main text, it needs to be configured so that it has permission to show the intro text to the public, but only displays the main text to users who have registered and logged in. On the right hand side of the Article Editor there are several parameter tabs. Under the first tab [Parameters – Article] there is an option labeled Access Level. By default it is set to public. Click the drop box and set it to Registered.

6. Click Save to save the article and exit to the Article Manager.

7. Select Main Menu From Menus tab.
8. Click on the Home Menu type
9. On Right Side
Parameters(Component)->Show Unauthorised Links; put the value as Yes.
10.Click on Save.

This article is for Joomla 1.5.x, you can if you want try it with older version, then please check this document