I can only open it as a website so I can't open the notepad to edit the document anymore. I'm a complete beginner, and I know the answer's going to be really simple, but I have no idea.
It sounds like you are doing HTML outside of the code academy environment. If this is the case, just right click and then select either "Edit" or "Open With >>" and choose the application of your choice to modify the code.
I recommend you ditch Notepad for Notepad++ or TextWrangler(Mac Only); they are light weight so not a lot of junk to distract a beginner but much more helpful features such as syntax highlighting and line numbering.
Have you tried to open notepad first and than the html-file inside the program?