Wordpress plugins from wordpress.org are free - some will have upgrades available for a price - but generally all are free.
In your admin section you just need to install and activate the plugins from the plugins section. If it requires an API key you will need to get this from the Wordpress site or from the developer of the plugin.
Generally I will do a google search for a plugin's functionality and once I find what I am looking for I will download it through my wordpress, install and test its functionality from there. Alot of the time it is trial and error with these free ones but if you have the time to learn PHP/JavaScript basics you can build your own.
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