Disk full - how to find heaviest folders?

I have a VPS on BlueHost. It's linux CentOS. I don't know Linux very well. I paid for an extra 10Gb drive but I don't know how to see which folders/files are being stored on which drives. For example I have:

Main /dev/vda1 - 65% used
Data /dev/vdc - 90% used

I had a fire drill tonight because my /dev/vdc drive went to 100% and I had all kinds of problems. Luckily I was able to clean up and delete some stuff, but that was just a temporary bandaid.

- How do I see which folders/files are stored on which drive?
- How can I control which drive my daily/weekly backups gets stored on? (I control my backups inside cpanel, and currently had to turn of my daily backups until I get my storage figured out). How can I move folders/files from one of my drives to another, without breaking websites and databases? (I have several hosted websites and 21 databases.)
- How can I see which folders on my server are using up the most disk space, sorted from top to bottom? A tech suggested "du -chs /* | sort -h", but I can't find how to run the Linux command shell from WHM in order to run that command. Is that the best command for this? Is there anything more GUIish so I can see visually what is taking the most?

Thanks so much for any advice. I am sorry I don't know Linux well.