How to change the content of the Joomla home page
If you have installed the sample data your frontend's home page will display several default featured articles (About Joomla!, Beginners, Upgraders, Professionals). If you haven't, the home page will be blank, but the Home menu item will still be set to display featured articles.
To change the default featured articles with a single page containing a body of text (and picture, or whatever you want), or to fill the blank home page, first log in to your site's backend and go to the Article Manager (Content>Article Manager).
Click on the New button to create a new article, name it whatever you want (e.g. Home), from the Category drop-down menu select Uncategorized, type the text you want to appear on your site's home page and save the article. Next, go to Menus>Main Menu and click on the Home menu item to edit it.
On the edit screen click on the Select button next to the Menu Item Type option and change it from Featured Articles to Single Article. Then from the Required Settings panel of the edit screen click on Select/Change and select the article you created earlier. Save the changes and refresh your frontend's home page to see the new page.
If you want you home page to display multiple featured articles but you want to replace the default ones, simply don't change the Menu Item Type of the Home menu item to Single Article. Create the articles you want to use as featured from the Article Manager. Then, if you have installed the sample data, unpublish the default featured articles.
The last thing is to make your new articles featured. This is again done from the Article Manager: mark the checkboxes in front of the articles and and click on the Featured button in the upper right corner.
For more information and screenshots check out the tutorial on changing the Joomla home page.