there are various software, but before you may need to define which fulfill your need. obviously you will need a browser, preferably main browsers like MSIE, firefox, opera, safari. if you like to code, then you can use a simple word processing like notepad or wordpad. there are more, but can't name all of them (i use pc platform).
if you are more the type of 'what i see, that is how i want it' (correctly said WYSIWYG = "what you see is what you get"), then you might want to consider products like DreamWeaver, GoLive, FrontPage. some free product are: Nvu, Mozilla Composer. there are lot more but don't know their name.
perhaps an image editor like Photoshop, PaintShop Pro, CorelDraw, GIMP.