for me it was the same way that i learned programming or guitar:

1. start with tutorials or poking at an existing design until it does what you want; learn the fundamentals of html and css from a dummies-level book or tutorials

2. find motifs or patterns you like, and reverse engineer (i.e. steal) them: specifically, start with a canvas with the original image on the left and try to create it from scratch on the right

3. repeat 2 until you have a toolkit of techniques (e.g. web 2.0 motifs like gradients, rounded corners, patterned backgrounds, reflections, all that cliched shit :)) and can put together things from scratch

4. integrate more formal theory (graphic design books, typography, photoshop/illustrator technique, web design books, etc.)

your first few designs will suck, but you'll get better :) but definitely learn by doing.