How you describe/introduce your characters varies depending on several factors. One is which POV your book is written in. There’s a lot of articles out there about how to introduce your MC when you’re writing from a first person POV. I’ve seen some books where the character introduces themselves (My name is Aivee, I’m fourteen years old. I’m tall and I’ve got dark blonde hair and yah dah yah . . . you get the picture. It can be done differently.) in the beginning of the book, but unless you’re MC is writing the book for someone, this is unrealistic. Most people don’t spend their time contemplating what they look like. A lot of people don’t like what they look like, so they aren’t going to spend all their time depressing themselves (unless that’s a big part of their personality. Remember that there are always exceptions to these general rules).
Wait for a chance for your MC to describe him/herself to come up naturally in the book. There’s a good chance it’ll happen without you forcing it. A lot of people say the most cliche way to have your MC describe him/herself is using a mirror, but this isn’t always true. For instance, in Mage, my MC has been travelling as a refugee for over a year and hasn’t seen herself in a mirror for all that time, so when she finally arrives to safety, washing off and seeing her reflection merits her reassessing what she looks like. You can describe your MC gradually by everyday actions and by them comparing themselves to other, new characters.
You don’t have to describe every character as you meet them – especially not when you’re writing in first person. Your MC isn’t going to describe his/her best friend because s/he already knows what s/he looks like. S/he doesn’t need to remind herself of it. I think it works well to gradually talk about what they look like as well. Add in lines – she ran her fingers through her dark, tangled hair is an easy enough sentence and it betrays a small amount of information about said girl. She has dark hair. Gradually build up what your minor characters look like.
This being said, if your MC is meeting someone for the first time, include what they look/act like. We say don’t judge a book by it’s cover, which applies to people, but we don’t follow up on it very well. Everyone makes judgments of people when they meet them (and if you don’t, congrats. You’re a better person than I am.) and so you’re going to take some time to notice what new people look like.
Because I’m a writer, I spend a possibly creepy amount of time looking at people. All of them are potential characters to me, so I notice small things the first time I meet people. (If you’re ever bored in a crowd, pick a person and describe them like you would in a book. It can be a lot of fun.) Think about your MC, though. Not everyone notices all the things writers do. The age and gender of your character can change what you decide to include about characters descriptions. What is the relationship between the MC and said character being described? This is where you can sometimes describe the main character at the same time. I can’t speak for guys, but I know girls will often compare themselves to other people.
Writing in third person is a little different. You can describe your characters to whatever amount of detail you want, and can describe characters that your MC has known for ages because you’re not limited to a single POV.
What are some of your favourite character introductions/descriptions?