if you’re not using any standard packages (portals, cms etc) and want to write your own form for users who don’t know anything about html to post html contents, one solution is to use tinyMCE, an open source (ie free!) javascript that transform <textarea> element into a wysiwyg editor.

it’s relatively easy to use, if you just want a simiple editor. more advanced implementations require a bit of learning curves. it’s well documented in its wiki pages.