What most people asking this question are looking for is a simple, tactical list of the hands-on skills necessary to work on the Web, a set of discrete tools to acquire that will outfit them for a new environment. More often than not, they feel drawn to Flash as a starting point, in part because it seems to allow the closest approximation of the print designer's pasteboard: the ability to specify virtually any typeface, a high degree of fine-grained control over the spatial layout of elements, an emphasis on visual invention, and a fairly straightforward way to animate normally static graphic design conventions.
Flash is wonderful, and I think it has it's place. But I think it's absolutely the wrong way to start learning how to work on the Web. It leads too easily to the assumption that a similar amount of authorial control can be exerted in online design as can be achieved offline - which is a fallacy.
It's far more expedient, to me, to learn HTML and CSS - the foundation for everything - and to learn how to code a simple Web page using those skills (rather than opting for a WYSIWYG tool like DreamWeaver).
Still, even these basics are the second step, in my view. The prerequisite for doing something meaningful with any of these skills - HTML, CSS, Flash or whatever - is first embracing the medium as something different from print. Indeed, there's no point in learning these skills unless as a print designer you've made a prior shift in your understanding of how design works in digital media. Specifically, come to grips with the fact that, on the Web, design is not a method for implementing narrative, as it is in print, but rather it's a method for making behaviors possible.
More often than not, the reflexive approach that I've seen print designers take on the Web is to treat it as a vehicle for print-based design practices: fixing type sizes, specifying typefaces, ignoring usability and expediency, and perhaps most notoriously making the assumption that, over time, users will come around to a print-focused way of consuming content.
In my experience, none of those tactics work. Their all-around ill-suitedness tends to boil over to frustration when print designers realize that, by and large, there's little room for visual virtuosity online. Which is to say, the Web is not commonly an effective tool for highly expressive displays of typographic, photographic or illustrative skill. Looking for opportunities to execute the sort of improvisational and dramatic creative visions that we see in printed periodicals, for instance, is likely to be an exercise in disappointment.
Rather, working online is very much about small details that build into a cohesive larger platform. I know that all design is like this to some extent, but it's especially true for the Web. The ratio of constraints to possibilities is far less kind in digital media, and understanding those constraints - understanding how to finesse them and how to subvert them appropriately - requires an attention to detail that bores all but the most dedicated.
There's plenty that's been said along this front already. The fact that I'm repeating that the Web is not like the printed page, that online information consumption is fundamentally different from print, is certainly adding nothing new to the conversation. The point I want to emphasize is not just that print designers need to be aware of this, but rather that they should approach it with genuine alacrity.
In learning a new medium, enthusiasm and open-mindedness trump nearly everything. A print designer who expects to succeed online, or even expects to master the skillset, has to be one who eagerly devours information about the medium, who peeks at the source code of Web pages that fascinate her, who spends her after-work hours experimenting with self-directed projects. This person needs to be motivated by the medium's possibility, and not solely by the fear of losing a job in print.
This is why a fundamental understanding of HTML and CSS is so crucial; such knowledge provides the means to begin experimenting, to begin understanding how a page is put together, how it is delivered to a browser, how it behaves and, crucially, how the designer's intention maps to how it is used by real people. Without that basic sense of curiosity, that insatiable desire to experiment and understand new ways of doing everything, the Web isn't much fun at all, regardless of how much experience a designer has under her belt. Just as is true with most everything in life, it's hard to get good at something unless you're having fun.
Khoi Vinh is the design director for NYTimes.com and the author and proprietor of a popular blog at Subtraction.com, where he writes about design, technology and culture.