W3 Schools do a very comprehensive guide on XHTML, HTML & CSS which you should look up also.
http://www.w3schools.com/html/default.asp (html)
http://www.w3schools.com/xhtml/default.asp (xhtml)
http://www.w3schools.com/css/default.asp (css)
But really it depends on how you learn best, I find learning easier by reading in a quiet room rather than on PC because I always get distracted on something!