Logging in the Joomla admin area (valid for Joomla 1.7 and 1.6 too)
First, in order to build your web site you will have to log into the administrator area. This can be achieved by opening the corresponding URL in your web browser:http://yourdomainname.com/administrator
Replace yourdomainname.com with your actual Joomla URL.
Next, you have to enter the admin login details which have been set during the installation:

Creating articles in Joomla
After entering the Joomla administrator area, you can check the featured articles (the ones that are included in the sample data or created by you) published on the front page through Content->Featured Articles.
If you have installed the Sample Data, before adding new articles you can delete the sample ones. Select them and click on the Trash button. Then remove the menu links to the sample articles from Menus->Main Menu.
After this you can add a new category from Content->Category Manager->Add New Category.

Next, navigate to Content->Article Manager->Add New Article.

Once you are ready with the settings you can proceed with the actual article content. Use the included text editor to type the article text, to include images or to directly edit the HTML code.

You can also adjust the settings in the Publishing Options, Article Options, Metadata Options menus.

Once you are ready, Save the article and check it on the front end. You can always edit the existing articles from the Joomla admin area->Content->Article Manager.