There are two easy methods for installing new plugins for your WordPress blog.  You can use the Search feature to find plugins in the official WordPress repository, or you can upload them manually yourself.  Note that just because a plugin is in the official repository is no guarantee of its quality, nor should you assume that plugins not in the repository are no good.

Installing Plugins Automatically

WordPress features a “search and install” method for installing plugins directly from the official repository.  In your WordPress admin panel navigate to Plugins on the left-hand sidebar, and then click Add New.

Enter a search term and then click Search Plugins.  A list of results will appear that match your search term.

Click on the Install link to the right of the search results for the plugin that you want to install.  A window will pop up with some information about the plugin and ask you whether you are sure you want to install.  Click Install Now to proceed.

When it will be install, you have to click on Activate for use.

The plugin is automatically fetched from the repository to your web hosting server and installed by WordPress.  You can now proceed with configuring the plugin for your blog by referring to the instructions that the plugin author provides.

Install Plugins Manually

Some plugins such as commercial plugins or those that are brand new and still being developed will not be available on the repository.  These plugins can still be installed manually on your WordPress blog.  Once again in your WordPress admin panel navigate to Plugins on the left-hand sidebar, and then click Add New.

Click on the Upload link at the top of the page.

Click Browse and find the Zip file for the plugin that you want to install.  Once you have chosen the file click Install Now.

The plugin is uploaded to your blog and installed by WordPress.  You can now proceed with configuring the plugin for your blog by referring to the instructions that the plugin author provides.