Research graduate school programs in psychology or counseling. Check each program's website for biographies of the staff and look for faculty members who have worked as behavioral therapists for autistic children.
Choose an undergraduate degree that incorporates the pre-requisites for your graduate program in its degree path.
Register for and take the Graduate Record Exam. Apply to the graduate schools of your choice.
Apply for work-study and research positions with faculty members who work as behavioral therapists with autistic children.
Apply for an internship at a facility that provides treatment for autistic children.