How can I know what is consuming my account disk space?

To find space used by each email:
1- Login to cPanel. 
2- Choose "Email Accounts".
3- In the emails list below, click the word "Usage". This will sort the list according to disk usage.
To find detailed space usage for your account as a whole:
1- Login to cPanel. (How?)
2- Choose "Disk Space Usage".
3- Click on any folder to get more details.