To add a plugin in wordpress, First go your login to your wordpress dashboard. On left side you see the Word Plugins, click on Plug in a pop up window will appear and click on add new.
After click on Add New you will search your key word. You can search your desired plugin by using a releated key word in search bar. when you search a key word you will see releated plugin about your releated key word.
click on your desirable plugin which you want to install. it take some time after that it will be installed.
After installation click on Activate to use the plugin.
Plug in will be activated as well
Click on plugin and then click on installed plugin, The activated plugin will be visible. Click on plugin setting and enhance your plugin setting with your comfortability.
After that you will be able to how to add plugin in WordPress. for more click here