You will need to know the database name, username, and password; these can be found in the database section of your config.inc.php configuration file:

[database]

driver = mysql
host = localhost
username = pkpuser
password = password
name = ojs

Back up the database using the mysqldump tool, e.g. if the backup should be placed in /root/:

$ mysqldump -u pkpuser -p ojs | gzip -9 > /root/ojs-database.sql.gz

Of course, phpMyAdmin or other tools could be used to back up the database.