If you are new to cPanel then you can check : Navigating around in cPanel.
Once you have logged into the cPanel, click on the Disk Usage icon under FILES 
After you have clicked on the Disk Usage icon, you will be able to see how much storage space is being used by each directory and sub-directory in your account.
Scroll down to see the entire list of directories and other details.
You can also sort your directories by Name or by its Disk Usage.
Disk usage details is useful in determining which directories are using the most storage space. Once you know where all your storage space is being used, you can decide whether you need to delete some files, or upgrade to a larger plan.
