There are several applications that will enable you to create a document in HTML format. Any one of them will do. Many popular word processors are able to save documents in HTML format. If your word processor does not have this option, you will need to use a different application to create the HTML pages. Some popular packages that allow editing HTML files include Front Page, and DreamWeaver. The application that you use isn't really important as long as it is able to save in HTML format.
If you have access to Microsoft Word, it has an option to convert a normal Word Document into the HTML format. The conversion isn't perfect, but it generally does a very good job overall.
Select Save As Web Page from the File menu.
Select "Web Page Filtered" as the file type.
Once you've completed this process, you'll have the appropriate HTML format document that you can then use on the website.