I'm not yet out at work, but I've been researching the correct process for coming out at my job. I've been reading various documents on my company's intranet specifically related to transgender workplace transition. There's a fair chance that none of my local managers are familiar with anything trans, but I work for a very large international company that does have policies in place.
From what I've gathered, the important thing is not to blindside your direct supervisor by showing up for work suddenly one day dressed as your true gender and demand to be addressed appropriately from that day forward. Give them advanced notice before you legally change your name, or tell your co-workers, or grow a beard, or whatever. You can inform your manager first, or HR, or your staffing agency, or request a meeting with both/all of them together. HR will need to know eventually for changing your name and gender marker on all their records. This ought to be treated as confidential medical information, and your manager ought not spread the word around the company without your permission.
Then what I was reading suggested that the entire workgroup be informed of the employee's transition in a meeting. The trans employee may or may not be present for the meeting. During this meeting, someone (not necessarily you) can explain what transgender is, dispel misconceptions, and lay down expectations of how you will be treated. This would occur on the "day of transition" or just prior to it. By this point, the company is responsible to ensure that the trans employee has a new id badge, records are updated, correct uniforms have been issued, etc.