Start by learning the latest trends, techniques used in modern web design. The do's and don't of web design, usability. Then learn from the best web designers, try a lot of tutorials related to the topic which you may want to learn and practice a lot, try looking for small projects, building your portfolio would be a lot fun and for your own interest.

Plus it takes a lot of passion and effort, if you really like what you do, you'll get good at it sooner or later.

Good luck!