just go to www.w3schools.com and look up the HTML section, XHTML is there too, that's basicly HTML but it's more in the style of XML. Just read trough the tutorials, first read HTML, then I recommend reading XML and after that XHTML and after that CSS. I know it looks like A LOT, but it's not. If you understand the basics, it's not that hard from there on.
After that you could maybe also try some PHP or JavaScript or something if you want to