you should probably start with xhtml, as you'll pick it up in about 10min. it's essentially html with some basic modifications eg everything has to be lowercase, and all tags have ending tags, eg
Code:
<img src="url.gif" alt="blah"></img>
except there's a contracted version of that that most people use
Code:
<img src="url.gif" alt="blah" />
the only other major difference I know about is they've gotten rid of some of the most painfully useless tags like <blink> and <marquee>