First - PFLAG is an excellent resource.
Second - Go slow and deliberate in any decision making. While your daughter has just come out to you, she's probably been working on this for a while. My personal opinion is that with the birth of a baby, things will be crazy enough and adding transition will just ratchet emotions up to the sky.
There are several stages to a successful gender transition. They overlap in many ways and not all have to be accomplished to transition. The first in my mind is "social". This is generally how someone deals with the outside world and how the outside world deals with the person transitioning. This can include visible changes - growing hair and styling in feminine ways, losing facial hair (electrolysis or laser - expensive). Wearing clothes appropriate to the new presentation. The list is endless, but you probably get the idea.
Second would be "legal". Changing legal name, birth certificates, job information, insurance, etc. All of the 1001 pieces of paper we are tied to. Some of this can be done easily, "Please refer to me as Mary", some is a lot more complicated.
Third would be "medical". Does she want to start hormones? Get body modification surgeries? How old is she and how much has she been affected by Testosterone. Adams apple? Brow ridge growth. Many more physical issues that differentiate male and female.
It's rare that a marriage can survive a gender transition. Not impossible, but unusual. Look up "Genderbread Person" for some explanation of concepts that will come to the forefront.
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