The letter depends on the therapist. While any therapist (or person otherwise qualified) can write a letter for T, many therapists without experiance in gender issues will refer you on to someone else that does have experiance.
Just because you are coming out to a therapist you've known for a long time, don't count on getting T right away. There is usally at least 3 months of therapy or Real Life Experiance to complete beforehand. This is flexable, of course - it's only guidelines and it depends on your situation. Assume they will want you to continue therapy for a few months or more before the T letter anyway, just to be safe (especially if you haven't mentioned gender issues as a kid, you're not "out", and not full time already).
I don't mean to dissapoint, I'm just saying it might take a bit longer to get on T than the impression you can get off the internet. Hell, most appointments to see the right people have to be booked weeks or months in advance. If you're thinking of T and transition - start talking to your therapist about it as soon as possible. Do not wait - communcate!
I went down the path of being out, full time, name changed and all that well and truly pre-T. I intended to have all that done 3-6 months before T, but it's looking like it'll be closer to 12 (appointment lead times FTL). It's a slightly different tactic, but it has its merit.
