Qualifications
You'll need a master's degree in psychology or counseling with a specialization in art therapy—or a master's degree in art therapy.
It's going to be a lot more expensive to find a program that has a degree specifically in art therapy (as there are fewer of these programs and mostly in private colleges)—plus you may have a harder time finding work if you are too specific with your degree.
Frankly, you're better off getting a psych degree with a specialty and then getting licensed credentials such as Art Therapist Registered and Board Certified Art Therapist from the Art Therapy Credentials Board (source). That will give you more flexibility in your career.
It also helps if you work well in a variety of settings, such as academic, hospital, and private practice. Knowing your way around a set of paints can't hurt, either.
Being good with all kinds of people is important: You'll need to be able to handle a variety of complex psychological issues.