You can browse the Statamic Marketplace to find addons, or use the "Addons" section inside your Statamic Control Panel.
Within the Control Panel, you can install addons by browsing to the one you want and clicking install. Behind the scenes, the addon will be installed using Composer.
If you want to do it yourself, you can use Composer directly on the command line:
composer require vendor/package
To learn how to create your own addon, as well as publishing it to the Statamic Marketplace, head over to the Extending Statamic area.
Addons may require a license, which you can purchase at the Marketplace. Licenses may be attached to a site in your account area. Make sure that you have your site key entered into your Statamic project.
An addon may have multiple editions, which may cost different amounts and provide different sets of features.
You can choose which edition is installed by entering it into your
'addons' => ['vendor/package' => 'pro',]
Or, by choosing it from an addon's details view in the Control Panel.