Your user name and password is the same as your computer login at the school.
Webmail is a CHSS email account that every student at school has. Consider using your CHSS email account for school-related projects and for more professional type of settings (on your resumé or cover-letter perhaps?)
Nextcloud is your "H drive" that you may hear some teachers talking about. When you save your work to your student account at school you are also saving it to your Nextcloud. If you need access to one of your files or projects from home (or anywhere else!) you can access it through your Nextcloud.
**note: it is always a good idea to BACKUP YOUR FILES frequently and often! You can do this by saving your files onto a portable disk/thumb drive, or by creating an online cloud storage account (like google drive or dropbox)