Let me start with an analogy, designing a website is akin to designing a car. In the case of car design, you will need to equip yourself with knowledge of Design (Of course!), Human Ergonomics (in case of Web Design, it’s Usability) and lastly have a good technical understanding of how the car actually works (in case of Web Design, it would be a general understanding the working of the web). So here you go:
Familiarize yourself with basic principal of design. There are plenty of resources online for you to start with Smashing Magazine: Design Principles: Visual Perception
Follow and study the works of designers from all field and not just web designers. Here is a good list to start with – Announcing The Winner Of Co.Design Bracket Madness!
If it’s not too late, get a formal degree in design.
And yes last but not the least, practice, practice, practice. If not anything else, recreate designs you like. You will end up learning a lot from it when you get into the mind of the designer.
Since Web Design is not only about design and not art, your work has to be understood and usable. Get yourself acquainted with theories of Usability and User Experience. You can start from here Usability.gov, Human-Centered Design: an Introduction or perhaps get a copy of Don’t Make Me Think, Revisited: A Common Sense Approach to Web Usability (3rd Edition) (Voices That Matter): Steve Krug
Tools of the Trade:
Using Photoshop should become second nature. It should become so effortless that you are not even aware you are using it and you are just focused on your design, that requires constant practice. Supplement your Photoshop skill with a Vector Editing tool, like Illustrator or CorelDraw. There is just something that a vector editing tool does better.
You will always have a more liking for one the three area, you can then focus on that and spend more time studying and practising that.