Instill hope for a positive outcome
Prospective clients are mainly focused on outcomes; they want to know that their investment of time and money is likely to yield positive results. Let them know what can be achieved by working with you. Of course it would be unethical to make false promises. However, when you decide to work with a client, you are no doubt hopeful that you can help them achieve their goals. Make sure that your website conveys this hope.
Include evidence where possible
When a website visitor lands on your site, it is important that they get the impression that you are someone who can help them. If possible, and if you consider it to be ethically appropriate, you could include testimonials or other endorsements of your work. You could also include case studies (real, composite or hypothetical) to give your visitors an idea of how your approach works. You may be able to think of other evidence you could use.
Highlight your unique selling point
Why would a prospective client choose you as their therapist rather than one of the many other therapists out there? You and your practice are unique, so draw out that uniqueness and highlight it in your copy. Even if you are a relatively inexperienced therapist, you will have particular areas of interest or some life experience which is unique to you and relevant to your practice. Find your niche. If you want to be noticed by your ideal clients, it is important to stand out from the crowd in some way.