Install extension using composer Magento 2

Follow below steps to install Magento 2 extension

cd <your Magento install dir>
composer require <component-name>:<version> --no-update (say mageplaza/module-core:1.4 )
composer update
bin/magento setup:upgrade
bin/magento setup:di:compile

You can install Magento 2 extensions by following way 

- Add an extension's package to the root composer.json by adding the component name in require section of composer.json. Say
    "require": {
        "magento/product-community-edition": "2.2.3",
        "composer/composer": "@alpha",
        "mageplaza/module-core": "^1.4"
    },
 - Then run composer update  and bin/magento setup:upgrade

- Now run bin/magento setup:di:compile and clear the cache