It depends what step you want to take.
Most doctors follow WPATH standards and they roughly require:
For HRT: one letter from a Therapist or equivalent (Counselor, Psychiatrist)
For most surgeries: (GRS or similar): 1 year RLE (Real Life Experience, aka living as your identified Gender) + Letter from Psychiatrist + Letter from Therapist or similar. Most surgeons also may want an all clear from your general doctor and maybe additional tests.
Name and Gender change in official documents really depends where you live. Some states only require a simple letter from your therapist while some have complicated processes and court requirements in place and some simply do not allow it at all.