Assuming that you are not looking for the miraculous quick fix (and I hope you are not), and you are pondering a real career change, then you probably need to think of it as such.
Learning HTML and CSS can be done in a weekend and you can make basic Web pages in no time. The problem is, so can anyone else, and recent software such as blog software and social networking software (like Ning) are automating much of this anyway. Then there are programs like Contribute that make Web page design about as easy as working in Word.
So if you are serious about this and want to pursue a career change, then you need to focus your energies on two key aspects aspects of Web design: tech skills and design.
First is technical skills. Do you have significant experience with Web technologies, from editors (such as Dreamweaver) to scripting languages (such as AJAX, Flash ActionScript)? Do you have experience developing interactive applications (MySQL, PHP, etc.)? Many developers specialize in one of these areas.
The second is design. Do you have a background in visual communications? Can you create a color palette, a wifeframe page layout, design comps, etc.? Are you familiar with usability practices and heuristics? Accessibility requirements?
Most Web designers treat one of these as primary and the other as secondary. Yet every serious Web designer I know does a fair amount of both.
So, depending on your timeline and needs, you can probably get low quality, low paying work very quickly with very basic skills. But if you can ramp up your skills you can get higher quality, more fulfilling, and better paying work.
There are books available on the market that teach you all of this–a trip to Barnes & Noble or Amazon is enough to get you started. But you might consider some more serious training, either from a local community college or even an official training provider for a Web technologies software publisher, such as Adobe or Microsoft.
Obviously, you can also do a combination. You can self teach the basic skills, just to get your feet wet and try to land a job or two, while also enrolling in some courses that will enable you to take it to the next level within a year.
I hope all that helps and good luck on your new career!