Install an extension using Composer
Before you can install any extension in Magento 2 you need to create a Magento Marketplace account. Then you can access you keys under the Access Keys section.
Once you have your Magento Marketplace keys ready, proceed to install the extension:
$ sudo composer require mailchimp/mc-magento2
The system will ask you for your Marketplace keys and proceed to download the extension.
Once this process is complete, update your modules as shown below:
$ bin/magento-cli setup:upgrade
Some specific extensions may require some additional steps, so it is always recommended to carefully check the installation instructions of the extension.