Upon a fresh installation of WordPress.org, whenever you attempt to install a plugin or theme, you will be prompted to enter your FTP hostname, username and password to proceed. To skip this part, you may save the information in your wp-config.php file.

First, ensure your FTP server is setup properly.

Next, go to the directory where your WordPress.org stuff are located (e.g. /var/www/your-website.com/public_html/. Make sure that the owner and group of all directories and files are set to apache (or that of Apache2). You may check from /etc/httpd/conf/httpd.conf. If it has not been set to apache, change it:
chown -R apache:apache /var/www/your-website.com/public_html/

Edit your wp-config.php file:

vi wp-config.php
Add these settings:
define('FTP_SSL', true);
define("FTP_HOST", "your-website.com");
define("FTP_USER", "username");
define("FTP_PASS", "password");

Try installing a plugin to test this out!