I am not an art director but I do tasks virtually that are similar to what Art Directors do in the real world - like creating a conceptual vision of what something will look and feel like. I am an Instructional Designer and I create courses that often have to weave together with a single vision - look, feel, colors, tone, etc. I am also in my life a writer and a photographer. And I believe that all of those roles that I play have the same creative core - seeing the world around you in a new way. So what do I mean? I mean go and do something or see something that you know nothing about. Learn about different styles of any visual media - movies, interior design, painting, music, clothing design, even walks through nature. Learn what you like and what you don't. Go back in time and study the history of those same elements. What were movies like in the 1950's? What were the popular color pallettes at homes in the 1980's? What is the difference in colors and textures between a desert, rainforest, wet lands, highlands? Each place you visit - whether in a book or in real life - is going to trigger ideas. And you may not use those ideas for years, but at least they are there. As a creative, the one thing that I would recommend is to be curious and always be learning about something that you know nothing about. Even learning another language can give you insights into different ways to communicate and how other cultures use language differently than we do in English. Some of the most beautiful phrases of love I have heard and understood in another language, a way of communicating that has not clear explanation in English.
Good luck with your career,