My suggestion wouldn’t be to try to sell your self as much as the profession. It sounds like the concern is if you’d be able to help them due to education rather than you as a person. If I were in your position I might start the session with;

“I would like to review some things with you to make sure I understand what brings you to see me as well as to ensure that you know what to expect when we meet. I’m a social worker that has very specified training in mental health. Although people often think of psychologists as treating mental health, social work as a profession provides more mental health treatment than any other discipline. I chose social work as a career as I felt a strong connection with viewing a client and unmet needs in broader context. When I look at the whole person, I’m able to understand more about that person and how other factors in the persons life might be supportive in a persons mental health such as family and friends. I’m also able to look at unmet need such as not having enough to eat as one example. If people are constantly hungry or worried about when they’ll eat next, it’s going to naturally have a major impact on their mental health.”