If you have a close friend who comes out to you as gay, bisexual or transgender, it is important to know how to support them. You may not understand their new sexual orientation straight away, but it is still a part of them and does not change your relationship. Make sure to include them in your plans and show them that you are happy for them.
You can start by asking them questions about their experiences, whether they have dated other men or women, or if they have ever felt attracted to another gender. They might be hesitant to answer, but it is an excellent way of finding out more about them and where they stand on issues like same-sex marriage or AIDS prevention.
It is also worth looking at their lifestyle and habits for clues. For example, if they are very careful about how they dress and groom themselves, it could be a sign that they are more feminine and have feelings for the opposite gender. This is a stereotype, and it is not always a good indicator, but it is worth considering.
You can also ask them what they believe about the origins of homosexuality. There is a debate in the scientific and nonscientific communities about whether homosexuality is innate or a result of environment and life choices. Your friend will probably give you some insight into their beliefs and where they stand on this issue. You should avoid probing them too much, though, as this can be stressful.