Generally, it’s best to check whether it’s safe first. Check your employer’s policies and attitudes. It is illegal (in the UK, and most European countries) for your employer to discriminate against you based on your gender identity/expression, but discrimination does still take place.
This is a sensitive topic. If you rely too much on your job, and losing it would be catastrophic (e.g. not easily being able to find another job) then you should definitely be more careful.
A more common strategy that some people take is to test the waters first, so to speak. This could mean being subtle with appearance changes, or gently discussing gender topics with colleagues and bosses, to see how well the topics are received. At the end of the day, it’s up to you whether you do it or how you go about it. Some people change their jobs after they transition.
If you’re in the UK, then your employer is bound by several anti-discrimination laws, most notably Equalities Act 2010. This page has some useful information on it, containing guidelines that your employer is duty bound to follow. It might be beneficial to show it to them, after you come out: https://www.gov.uk/government/uploads/system/uploads/attachment_data/file/503663/Workplace_Guide_CSEP_revised_Final_V1_.pdf
We really don’t want to provide any specific advice, because everyone’s situation is different.
This page contains some useful information: http://www.tgender.net/taw/tggl/checklist.html
This page lists some of the very real consequences that can result from coming out as trans in your workplace (depending on your workplace): http://www.hrc.org/resources/coming-out-in-the-workplace-as-transgender
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