There are really 5 fundamentals you'll want to practice if you want to dive into user interface design for the web:

1. Typography

2. Whitespace

3. Color selection

4. Image use and presentation

5. Animation / iconography

With those 5 you'll get the 80/20 of what's needed to make a great design. From there you can dive into the specifics of user experience, information architecture, and a whole host of other facets to make things both aesthetically pleasing and intuitive.

If you want a crash course in those 5 topics, I actually put together a free book with examples on how to apply those things to the web: http://anonconsulting.com/ui-crash-course/

Let me know what you think and if you have any further questions don't hesitate to reach out.