I'm not sure what you are trying to do — to "save text as html" in a graphics program that is not an html editor is about as meaningful as asking how to use a refrigerator to save wheat as pasta. If you have the text as text, just copy it into the html if you want text. If you want to create a graphic representation of the text, Photoshop can help with that by saving image files that can be used in an html file. And you can use Save for Web and Devices to make an html fragment. However, Photoshop is not an html editor and cannot do much about formatting or laying out the text in html.
make a Slice, double click on it with the Slice Tool and select No Image from the Slice Type dropdown and enter your text. You won't see what you typed until save out the file or preview it in a browser. When saving/previewing the file, go to File->Save for Web & Devices, double click on the slice containing the text with the Slice Select Tool and make sure that Text is HTML is selected (it should be by default)
Well, you can using the method described, but generally I wouldn't do it. You have no control over the style attributes and the code the saver creates is not particularly elegant. If it is meant to be used for a web page, you run into any number of problem. Therefore, if at all, I'd only use that feature for local viewing, e.g. an intranet HTML presentation or such. The rest is better done with Dreamweaver...