First go to Ubuntu 16.04 initial server guide, https://www.digitalocean.com/community/tutorials/initial-server-setup-with-ubuntu-16-04.
After done with initial server setting, I need to install LAMP environment, https://www.digitalocean.com/community/tutorials/how-to-install-linux-apache-mysql-php-lamp-stack-on-ubuntu-16-04.
Install these packages as well, sudo apt-get install php-bz2 php-curl php-gd php-imagick php-intl php-mbstring php-xml php-zip.
After installing and setting LAMP, I need to manage self signed SSL, https://www.digitalocean.com/community/tutorials/how-to-create-a-self-signed-ssl-certificate-for-apache-in-ubuntu-16-04.
After this I tried to install OwnCloud, https://www.digitalocean.com/community/tutorials/how-to-install-and-configure-owncloud-on-ubuntu-16-04.
I did not manage to know for sure what was the cause of an error that makes OwnCloud could not connect to MySQL database.
The error code was like this, SQLSTATE[HY000][1045] Access denied for user 'user_test'@'localhost' (using password: yes). Some people told me that I might put wrong credentials (password, database name, ...). However, I can successfully login using MySQL interactive shell. Hence, here are two main possibilities for the error.
* MySQL server was turned off.
* The special characters in the MySQL password preventing the OwnCloud to login.
The error was happened because the MySQL server was disable. To enable it execute this command, sudo systemctl start mysql.