What's an ally? How can I help a person transitioning?

I'm a volunteer counselor for The Trevor Project which is a lifeline for LGBTQ+ teens and young adults, many of whom are considering suicide but some who just need a friendly ear. Trevor also has lots of guides and resources that may be helpful, as the previous poster mentioned.

There are some great suggestions in this thread. I would also add that you can use a subtle signal to show that you are an ally. For example, I wear a pride band on my Apple Watch during June, and a small pride flag pin on most of my clothes the rest of the time. LGBTQ+ people are very attuned to these small symbols and I hope that just by doing this in public it can make someone feel less alone.

If you have an office where people see you, put up a small sign that says something like "Everyone is welcome here." Make people comfortable by saying "you can talk to me about anything" (if you truly can).

Here's the link to The Trevor Project https://www.thetrevorproject.org/

That's a very good question! In my experience, an ally is a anyone who stands up for the right an individual has to be their authentic self. Sometimes its just lending an ear to the likely mood swings an individual may be facing on HRT, for instance, or a shoulder to cry on when life seems out of control. And in extreme cases, intervenes when an emotional, mental or physical threat is apparent or perceived. An ally offers an affirming comment such as "You look great in that outfit," or "Great new hair style," as an individual experiments with a new look to support their gender identity.
I am blessed to be married to my greatest ally and to be surrounded by loving family members, friends and business colleagues who may not fully comprehend what I'm going though, but are there for me whenever the need arises. They are all allies!

I will also add that being an ally means speaking up, even if it's slightly uncomfortable to do so, even when you think there might not be any LGBTQ+ people around. For example, a lot of young people still say "that's gay" when they don't like something. A response like "you know, when you say 'that's gay' you can hurt someone else's feelings, maybe just say 'that stinks' or something like that."

This is hard to do sometimes, especially if you're among friends and one of them starts saying anti-gay things like "they want to groom your children." Keeping silent is tacitly agreeing, so I will say something like "That's not true. The gay people I know all date other adults" or, if you feel that's too confrontational, just ask "Why do you say that?" to open a dialogue with the person.

Another thing that shows allyship, not just with LGBTQ+ people but with anyone who is different from the majority group is to be sure to invite them to things. For example, a couple of my black friends say they never get asked to lunch by their co-workers not because those co-workers are racist but because their co-workers assume they want to "hang out with their own." I've done stuff like this when I used to work in an office (am remote now): "Hey, Jim, we've worked together for awhile now. Would you like to have lunch?"

Completely agree. You've added some really good points. It all boils down to what I learned in Kindergarten: Treat EVERYONE like you like to be treated. No more, no less.
I personally wear a ring daily in the transgender flag colors (a gift from my spouse when I came out). Clients see it easily. Some ask about it. Others are unclear of its meaning. In any case, I'm proud of who I am and will take the time to speak with people who are curious, as long as the questions are an honest inquiry to learn more.