I taught myself 7 or 8 years ago now. There’s no shortage of learning materials online and truly all you have to do is practice. I read a book on HTML and took and online class about how to use PaintShop pro (because it’s cheaper than Photoshop).

My favorite sources today include:
http://www.htmldog.com/
http://www.w3schools.com/

Good luck, and as with most things, you’ll get as much out of it as you are willing to put into it. It took years of hard work and learning web standards and some programming, but I’m now the webmaster/web developer for a large architectural firm in Seattle. I love it!