So today in tutorial I learned that CAS students have access to their own domain on the CAS website - but there isnt really any information on how to get it set up. I know I can access it through Putty and put files onto it from Filezilla, but I have no idea how to make it accessible or where to start. I'd ideally like to start my own website and possibly host it there.

As far as I know, there's nothing you can do to "make it accessible." Just create a directory inside your own home directory called "public_html" and it may become accessible via eg http://cas.mcmaster.ca/~mattinui. If it is, then you can just drop your HTML and whatnot in there and treat that directory as the root, just like eg /var/www/html on some other Linux distros. Right now it seems that the webserver is denying access, at least for my account, but I've had it working before. It just depends on how they have the server set up.