You start by learning all you can about people - and by studying how other authors portray people as characters. The two most important ways to prepare are to:

1. Read, read, read!

I said this before, I know. I'll probably say it again. Read widely and voraciously. Read the best that you can find—not just in science fiction and fantasy, but in all kinds of literature. If you like comics, read those, too. (Note I said "too," not "instead of.") Read fiction and nonfiction. Read plays. Read poetry. Soak up all the understanding that you can of what makes good characters, and what makes characters interesting in stories. Think about the characters you like, characters that touch you. Notice how the authors handle dialogue, motivation, and emotion.

Read some fluff too. And while you're at it, read some mediocre or even outright bad stuff. It'll help you learn to see the difference.

To get you started, I've provided a recommended reading list of some particularly outstanding SF/F. But it's only a launch pad; there are as many good stories out there as there are stars in the galaxy, and you'll find many other favorites of your own.

In addition to reading, you must . . .

2) Study people!

Study the people in your own life. Remember what I said in the section on ideas? Look at people all around you, wherever you go. Stop, look, and listen.

When you need a character in a story, borrow a few personality traits from your friends, or your parents, or teachers. Or that person who annoyed you in the music store. Or the sad-looking waitress who served you in a restaurant. Then—and this is important—don't just copy them, but take some of their personal quirks and change them around and make the characters your own.