The W3C Schools site is excellent for the self taught. Don't let the 'schools ' bit put you off - its really comprehensive (XHTML, CSS, JavaScript, XML etc.)
http://www.w3schools.com/
Also nothing wrong with notepad - although you might find that a proper editor will make it easier to organise your code - different colours for commands, set indents etc. There are loads of free ones out there:
emacs (http://www.gnu.org/software/emacs/)
jedit (http://www.jedit.org/)
and many more.
The W3C validator mentioned by Flash477 is also a must if you want your pages to display well on different browsers.