There are a couple of reasons why you will want to add a therapist to your transition team:
1) Transition is stressful and having a safe space to talk about your problems and feelings is extremely helpful, and
2) you will most likely require an assessment and letter of recommendation from a therapist before you can proceed with HRT and/or surgery. While some surgeries can be done on the basis of informed consent, such as Hysterectomy and Top Surgery, most surgeons will not perform genital surgery on a patient without the recommendation of a mental health professional.
Seek out a therapist who has experience working with transgender patients and is someone who you feel you can confide in. Before engaging a potential therapist, make sure they have written letters of recommendation for HRT and/or surgery before, and find out how many sessions they typically require to provide the letter. (This can vary widely: some therapists require a set number of months for therapy, while others will write a letter after a single session.) There are a number of web pages that list reputable gender therapists. You can also ask friends in your support group(s) and other members of the trans community.