You need to accept that you do not have a son, you have a daughter. You also need to do a lot of learning about trans people so you can support her as she deals with the necessities of her life. Here, have some information:
http://www.t-vox.org/index.php?title=Trans_101
http://www.t-vox.org/index.php?title=My_friend/relative_is_transsexual
http://ai.eecs.umich.edu/people/conway/TS/Evelyn/Evelyn.html
http://www.ctys.org/wp-content/uploads/2013/06/familiesintransition.pdf
http://www.mermaidsuk.org.uk/
http://gires.org.uk/
Therapists have been mentioned. Here is a list of therapists who are experienced and open to working with trans people: http://www.drbecky.com/therapists.html
And these ones are for her:
http://www.t-vox.org/index.php?title=I_think_I%27m_transsexual
http://www.trueselves.com/forum.php
http://www.wpath.org/uploaded_files/140/files/IJT%20SOC,%20V7.pdf