Disk is full. Can't clear it. Help me please

Hello, When I try to create or modify an account I get the following error: The following disk partition is almost full: /. You must remove unused files in that partition before proceeding. Here's what I have already tried: - Increasing /dev/mapper/centos_static-root (I've had only 4 MB free, and not sure how to deacrease /home to get more space for increase) - Moving SQL files (however I'm not sure if it was successfull) - Opening /dev/mapper/centos_static-root by FTP to see what's inside (can't open this directory) So I am lost. Please help me.