I took two digital art classes in college. You could tell who struggled with design and who struggled with the tools. The kids who struggled with the design piece had really crappy images. The ones who struggled with the tools had really simple images.
You can learn either/or or both. I focus on really simple design because I don't know photoshop, css, xhtml, etc well enough to pull off complicated designs. However, using simple tools like size, layout, and color you can do a lot.
I recommend getting started with blueprint css framework and going from there. You'll learn more about design than the tools, but you'll also pick up some css.
As far as self-learning vs mentor/teacher goes. You need feedback. If you can get feedback while self-learning it can work. I know there are some websites out there that can help you with that.