How can I support my friend who is transgender?

Honestly, let them know that you are there for them. Ask them how you could help. Very likely, there won't be any specific thing that they tell you. In which case, just love them and listen to them and be their friend like you've always been.

Just accept them for who they are. They are still a person, and please call them by what gender they identify by.

Ask your friend any questions you might have. It's better to ask and get answers than to assume anything. Just remember to be respectful.

one great day to support them is to use their correct pronouns and always introduce them by using their correct pronouns. that will help with their gender dysforia

You can support your friend who is transgender by being there to support them and listen and understand what they are going through in their time of need.

I have a friend who is transgender, I think the best thing to do is just try to support their decision. Be there for them, talk about it if they want to talk about it. If you don't know how to treat them, or you feel maybe you should change your behaviour, ask them. It's probably better than you just trying to guess how they want you to behave.

Simply tell your friend that you support their choices do dress/act/be as they feel they should be. Also be there to listen to them when they need someone to talk to. And most importantly, show that whether their being transgender is a new concept to you or something you've known about for a while that it doesn't affect the fact you still care about them as a person. Also, just my experience with transgendered friends, always refer to them with the pronoun (he/she or the male/femalename) of their preference/choosing.

Ask them. No two people are the same. Some prefer active support and championing, others would rather you just keep treating them as you always have. Don't try to guess their feelings or desires. Just say "Hey, I know you're going through this and want you to know I'm here for you. What, if anything, would you like me to do to show my support?"

Be there for them just like you would any other friend of yours. They're slightly different than other people, but deep down they're just like anyone else, that's the kind of treatment that they would deserve.

You can listen to them and respectfully use whatever pronouns they wish. Do not try to tell them how they should feel but accept their feelings as valid. Treat them as you would treat anyone who may be struggling, and try to give them a sense of comfort/hope in relation to their situation.