in cpanel, there's a file manager. Within that file manager you can create new files, and there is an editor to use for filling the file with data.
However the easiest way (in my opionion) is to use a good text (or php) editor (I use notepad++ for both php and html editing) locally and then upload the file using ftp or filemanager. Minor changes are ok for doing in the online texteditor, but it's nothing I wanna use for coding the entire site.