The easiest way of installing a WordPress plugin is to use the plugin search. The only downside of this option is that a plugin must be in the WordPress plugin directory which is limited to only free plugins.
First thing you need to visit the Plugins » Add New page inside your WordPress admin area.
You will see a screen like the one in the screenshot above. Find the plugin by typing the plugin name or the functionality you are looking for. After that, you will see a bunch of listings like the example below: