Content Warning: this page includes discussions of coming out, invasive questions.
Please respect boundaries.
If someone comes out to you as trans, you might have a lot of questions. That's fine - but be aware of what's appropriate to ask and what isn't.
Coming out is a scary process and often happens at a time when the person coming out is not sure themselves what is going on. So it's best to check whether someone is okay with discussing their experiences more deeply before you ask probing questions, and be content with not asking if they're not comfortable.
Trans people spend a lot of time and energy educating the people they meet about gender and being trans. This can lead to some great conversations, but can also get pretty tiring if you're constantly having to explain stuff. If someone close to you comes out, maybe consider looking at some resources online to educate yourself on the issues involved, instead of asking them all the questions you have about being trans.
Some people are curious about trans people's genitalia, medical and hormonal status, and the way in which they have sex. While some trans people will be comfortable discussing this with their friends, many won't - so please never assume they are and never ask about these things without first checking it's okay to do so. Please also don't ask someone these intimate questions unless it might be a normal thing to ask within your relationship. Trans people get asked a lot of intimate questions by strangers, and it can be really uncomfortable.