Don't worry about asking questions! We all have been at the stage you are at of being new to this!
The diskspace is to do with the files stored on your server i.e. the files that make up the site. So the images, webpages, downloads etc. which are stored on the server. From what you said above, this is "depend on how big my
website hosting"
Everytime someone accesses one of these files, your bandwidth usage is what is used up here. If they accessed a 1MB file, you would use 1MB of your monthly bandwidth. Again, from above this is to do with "how many people visit it" but also what files they access e.g. a 1MB file uses less bandwidth than a 5MB file.
Looking at your site, it appears to use PHP which I would think you can edit from the editor in cPanel. Out of interest, how do you update it currently? Do you send someone a picture and the text or an actual web page?