If you use a CMS like Wordpress (there are others), it is just a matter of copy and paste. If you are using the traditional method of writing the pages yourself, HTML editors generally include an option to convert text files to HTML code. Of course, we are talking of very simple text files (just pure ASCII). If you want to include word-processing features, like bold or italic text, then you must get a conversion program. If you are not a purist (one of those that code with Notepad) and are not too much concerned with optimization, main stream word-processors let you "save as" HTML.