Go to http://wordpress.org/extend/plugins and search for the plugin you want using relevant keywords. After you have found some plugins, just check their stats to ensure their quality and also check the compatibility with your version of WordPress.
Download the plugin and save it on your computer.
Next, log in to your WordPress Admin Dashboard and click on the Plugins tab. Click on the Add New button.
Select the Upload tab and browse to the plugin you just downloaded and click on Install Now.
The plugin will be uploaded and installed. After the installation is complete, just click on Activate Plugin. Your plugin is now activated.