From the backend area of Joomla, mouse over, Extensions, and then Extensions manager.
The Extensions Manger Install screen will appear.
The easiest option for installing a new extension is to use the Upload Package File option. Simply clicking Choose file button and navigate to the extension that you have downloaded on to your computer.
Click Upload and Install once you have selected the file and Joomla will upload, extract and install the extension for you.
Installing from Directory
In other circumstances, you may have to upload the extension that you are installing to the file system of Joomla. You may do this via FTP or another file access and transfer protocol.
From here you will need to provide Joomla with the file location of the extension and Joomla will extract and install the extension for you.
Installing from URL
This option allows you to specify a URL where Joomla can download the extension to extract and install. This method won't always work as some extensions will require registration and account access to download files.
Where Can I Find Extensions
More extensions for Joomla can be found in the Joomla Extensions directory