First connect to your CentOS VPS using an SSH client like Putty.
A simple command to see how much storage folders are using is: du -hsc *
You can use this at the root level "/" though often storage is used by a folder under /var /usr /tmp /home, or /root.
It's often quicker to change to one of these directories first, for example using command: cd /var
Once you see a directory using a lot of storage change to that directory using the cd command and enter du -hsc * again to see what storage the directories under it are using.
To see a listing of files and their sizes in the current directory use command: ls -alh | more