When introducing a main character for the very first time, what would you say qualifies as information that the reader needs to know?
Depends on your story, to be honest. In McCarthy's The Road, we are told neither the man or the boy's name. But for the story, it didn't matter. So say you're writing a spy novel, you would need to know their height, build, and possibly ethnicity, depending on the type of spy novel you're doing. If it's a romance, I would focus more on hair, eyes, lips, nose, etc. Try and work out what readers need to know. Is the character going to bump his head on a doorframe in a few paragraphs? Let the readers know that they are tall. And on it goes. Hope this is of some use to you.
Anything that is naturally revealed by the events of the scene. Don't info dump character descriptions. Now, if you want to know what needs to be revealed in the scene, that's up to your particular story.
Gender? That's one basic thing every reader will guess from the moment you start using he or she instead of a personal name For a transsexual character - I have no idea. I'm yet to dwell into trans-g storywriting... (that's actually a freakin' good idea for my next story!! ) Other things: reaction of other characters when the character is introduced; his/her reaction to other characters; his/her reaction to a situation. Those three plus gender - I think that's quite enough for a character to stick out as an individual. All other details are either genre specific, or your own stylistic choice.
When I introduced one of my Main characters the reader knows nothing about him till a chapter or two after. It's all about what you prefer but also on what will keep the reader hooked.
Both in writing and in reading, my preference is to give details about characters as it's natural to do so. For example, in one of my novels the main character isn't physically described until she's sitting in a train looking at her reflection in a window and hoping that her make up has held up after just meeting a guy she likes. I didn't want to dump information of the reader too quickly.
I'm working on the best way to describe one of my lead characters. He's a little person and the POV in the first chapter. He's occupied with thoughts about an accident his daughter was just in - the scene is in the ER. So, I didn't want to just mention his dwarfism because he's not thinking about it. But, I also didn't want the chapter to get too far and have the reader surprised. It's important, but at the same time not a big deal. Right now, I wrote about his daughter being taller than him as a way of describing him. I'm sure I'll change this around before I'm done.
Don't forget that the way others react to him can also provide information to the reader. People looking down at him as he passes, comments behind his back to the effect that the circus must be in town, etc. Not everyone will be sensitive or respectful.
When entering any scene the reader wants to know where they are in time and space. They want to know what's going on. They want to know whose skin they're wearing. They want to know the protagonist's scene-goal so they'll recognize when something interferes with it and brings tension into the scene. But they don't want to be told those things by a disembodied voice (or the author wearing a wig and false mustache, pretending to be the protagonist at some later date), they want to observe them in real-time, often by knowing what has the protagonist's attention and how they respond to it.
I'd say put the character in a situation where we get to know enough about him to get interested without _telling_ us these things. The reader often don't need so much plain data about a character in that first chapter, as long as you make them interested. Sneek in that basic info a little at a time.
