List partitions and usage

df -h

Check Disk Usage per User

repquota -a -s

Show only accounts with usage in Gbytes

repquota -a -s | grep G

Check Disk Usage per Partition ignoring links
* You can replace /home with the parition or folder you want to check
Let's check /home partition

du -hx --max-depth 1 /home

Show only folders with usage in Gbytes

du -hx --max-depth 1 /home | grep G

Check /var partition

du -hx --max-depth 1 /var

Show only folders with usage in Gbytes

du -hx --max-depth 1 /var | grep G

Check Mail folder /var/vmail

du -hx --max-depth 1 /var/vmail

Show only folders with usage in Gbytes

du -hx --max-depth 1 /var/vmail | grep G

Check MySQL folder /var/lib/mysql

du -hx --max-depth 1 /var/lib/mysql

Show only folders with usage in Gbytes

du -hx --max-depth 1 /var/lib/mysql | grep G

Check Backups

du -hx --max-depth 1 /backup

Show only folders with usage in Gbytes

du -hx --max-depth 1 /backup | grep G

Show only daily backup folder with usage in Gbytes

du -hx --max-depth 1 /backup/daily | grep G

Now when you know how to check the partitions and folders here are some useful commands to check files in the folder sorted by size:

ls -lahS /folder/location

Example for MySQL folder /var/lib/mysql/

ls -lahS /var/lib/mysql/

Some useful scripts we already have are checking the most frequent folders:
/var/log/
/usr/local/apache/logs/
/usr/local/apache/domlogs/
/usr/local/cwpsrv/logs/
/tmp
/root
/var/lib/mysql/

/scripts/disk_check

Delete Files
Now when you know how to check, the question is how can you delete or empty files.
Note that you shouldn't delete files other than logs or backups as all others can crash your server.

Log files are in use by the programs so in case you want to clean big active log files you can't just delete them as they will remain invisible until you restart the service.
The solution is to empty the file, example:

:> /usr/local/apache/logs/access_log

However all other logs files you can simply delete, examples

rm -f  filename

An example case when you have active log files and logrotated which are not active and are renamed with -DATE

Delete single log file NOT currently in use by service

rm -f /var/log/maillog-20211108

Delete all maillog files NOT currently in use by service

rm -f /var/log/maillog-*

Empty active log file which is in use by the service

:> /var/log/maillog

* If you are not sure it's better to grab a support service and have the experienced sysadmin to check this for you.