Starting HRT (Hormone Replacement Therapy) is usually done through a gender therapist who refers you to a knowledgeable doctor. The sooner you start hormones the betteras the body ages it becomes less receptive to hormonal changes.
You can start HRT and still work as a guy (or woman for the FTM) for a while, but eventually the changes will be noticed by your co-workers and customers, like a deeper voice in the FTM (that happens fairly rapidly on testosterone and is permanent) and breasts and other physical changes if you're MTF. If you can dress causally at work loose clothes can hide breasts. FTMs use a binder before a bilateral mastectomy to conceal the breasts.
If you haven't already you'll need to start electrolysis or laser. Electrolysis is still the only FDA recognized permanent hair removal method. Laser has been approved for permanent hair reduction by the FDA. Whether there's a significant difference between removal and reduction is debatable. Laser is reportedly faster, less painful and can cover larger areas than electrolysis. Some trans women clear their faces with laser then "clean up" with electro. Electrologists and laser methods and technicians vary in price, type, skill and technique. In the beginning, you'll need to have some beard growth for the electrologist, or laser technician, to work on. This can make living full-time difficult, especially if you have a dark beard. You can have electro/laser done while still working in guy mode or living part-time.
The decision to transition at your current job, or start fresh at a new employer, is an important one. Some of the more progressive companies, like Intel, Microsoft, IBM, etc. have policies regarding transitioning. Check your employer's anti-discrimination policy. Also, check your local anti-discrimination laws to find out if gender identity is included.
Where you live and how well trans people are accepted in your community often effects how you're treated. You may encounter discrimination and harassment on the job in a conservative area and even some liberal ones. Unfortunately, it's still legal in most states to fire someone just for being transgender.
If you want to transition at your current job, I recommend writing a letter to your employer stating you've been diagnosed with GID, or gender dysphoria, and "as required by the Harry Benjamin SOC (Standards of Care) will be transitioning to the female/male gender." Explain it's a medical condition and you're following the standard procedures (i.e. hormones, electrolysis, living full-time, etc) in preparation for your SRS. A letter from a therapist should be included.
The bathroom issue can be touchy. It's probably a good idea to get your fellow employee's thoughts about the restrooms and find out if there are company policies in place regarding them. Some employers offer unisex bathrooms or other accommodations, but there's a fine line between reasonable accommodation and discrimination. Once SRS has been performed it's generally accepted that you can use the restroom congruent with your self-identified gender.
Sometimes it's better to start over at a new job (especially if you're very passable) and avoid potential harassment, wrong pronoun usage, or termination.
Once you're close to living full-time it's time to change your IDs to your new name and gender. Usually the first step is getting a court-ordered legal name change. The procedures differ by state, but typically you file a form with your state civil court, pay a fee and are given a court date. Changing your driver's license also varies by state. Sometimes a letter from a therapist is required (this can be useful for other ID changes as well). Some states will let you change the sex on your driver's license before SRS, others require you to have completed surgery. For Social Security, call your local SS office for details on changing name and gender.