100% disk full on Centos?

Can someone tell me, why is this 100% used on the /?

How can I fix this problem? I'm not a unix admin, I'm a programmer but can read the books.

[root@datasrv /]# df
Filesystem           1K-blocks      Used Available Use% Mounted on
/dev/sda2              7757352   7757248         0 100% /
/dev/sda1               248895     15027    221018   7% /boot
/dev/sda9            120098680   3173944 110725656   3% /data
tmpfs                   777052         0    777052   0% /dev/shm
/dev/sda5              3882508    128136   3553964   4% /home
/dev/sda7              1945132     35888   1808840   2% /tmp
/dev/sda6              7757320   4728956   2627948  65% /usr
/dev/sda3              7757352    571420   6785512   8% /var

Follow up:

I deleted some files then ran my rsync program which syncs from a windows mount share drive to a USB drive. It doesn't write to the local Centos disk as far as I know. Then suddenly the disk drive is full again, why? Does rsync do something that creates lots of files somewhere?

Find big files and remove them if possible:

find / -mount -size +8096 -ls
find / -mount -name core -ls
du -sh /* (and repeat for subsequent dirs on /)

Probably something is flooding your / or you have to many old kernels. Cleaning up will help, your / should be big enough because you've a seperated /var and /usr (and /data ...).