Ask about their training and background. Describe the kinds of issues you want help with. Make absolutely sure that the therapist is familiar with ADD in adults. They should be knowledgeable about ADD and its manifestations so they can help educate you and help you realize your difficulties in life were the result of subtle neurological deficits in the brain, not character defects or moral shortcomings. They need structure and guidance to stay focused, to stay on topic, to get the most benefit from their therapy If you find a therapist that seems good via the phone, set up a longer, in-person, interview.
Some therapists provide an initial half hour session at half their regular rate.
Others expect you to devote a full hour to getting acquainted and seeing if you can work together. Pay attention to your feelings during this initial session. It's important that you find someone who makes you feel comfortable and who gently challenges you to explore your inner life and past experiences. If you feel the need to lie or withhold important information from your therapist, you are not going to get any real help. When you find a therapist you like, plan to give therapy about six sessions before deciding whether or not to go on with it.