I have transitioned twice on the job and learned the following:
1. Do not trust any managers, go to HR and ask that they help you with it and help get the word out.
2. Get EVERYTHING in writing.
3. People will be jerks and some may even end up needing to be fired for sexual harassment.
4. Use your resources efficiently and effectively (therapists, etc)
The first time my manager said that he would handle everything. I walked in my "first" Monday morning to find out that he had not told anyone. He felt it was not his place and that my showing up would be the best way. Yeah, went over like a lead balloon and I ended up with no real work being finished up because everyone had to come and see the new me. Fortunately, I had outed myself to a select few friends there and they were a tremendous help. It was this job (retail) in which a coworker ended up being canned due to rude remarks of a sexual nature. I was told I would be fired if I went near the women's room. Joy. Fortunately I had a private bathroom I could use at work.
The second time around there were resources on the web and I used them to the fullest. I bought two books for work (the managers and HR person read them) and outed myself to the head of HR first and let her handle a lot of it from there. Multiple meetings after that and a time line was set down. I had to give up requesting that management include a note of support with my letter to the employees in exchange to being able to use a specific women's restroom (until post-op). A lot of wheeling and dealing took place and I stressed my safety as my main concern on most issues. Things have gone about as well as I could have expected. Did have one or two people a bit upset that I had not told them beforehand.
The state I live in is not friendly to Ts. The place I work for is very political but I used that to my advantage.
Of course YMMV and good luck.