As far as I’m concerned you can’t read too much about the topic of voice in your writing. At least not until you actually get it.
If you watch TV, you see examples of character voices all the time. Let’s say the American TV show Bones. You have Dr. Brennan. She’s very rational. She’s a little clueless about pop culture and normal social behavior. So she says a lot of brainy things that very few people understand. And then there’s Booth. He’s not stupid, but he’s not as smart as Dr. Brennan. Everything he says arises out of his character. He’s sympathetic, but he’s been a winner all his life. He takes certain things for granted. He was a jock. Every one of the characters in the show has a distinctive way of looking at the world, a distinctive way of talking. This is true for all good television, movies, and books.
In fact, I’d go so far as to say that in books, it’s even more important, because in a TV show characters get some of the voice from the actors who portray them. But in a book the reader has to get the character out of the words.
Here are some things that might affect a character’s voice:
Their age. their generation, the time period, where they grew up, where they spent their young adult life, their socioeconomic level, their education level, their native language or languages, their ethnic background (for example if they’re first generation Americans and their parents spoke another language at home or if their cultural practices at home didn’t follow the ‘mainstream’), their gender, their birth/growing up family size, whether they came from a two parent or a one parent household, their intelligence, their mental health, whether they are introverted or extroverted, their values, their core beliefs, their personal history, their job history, their relationship status, the social groups they hang out with (such as geeks, nerds, science students, equestrians).
You can find a lot of other things about your characters that might shape their voices. Now, voice doesn’t just mean how they talk. It means how they see the world. If you’re writing in first person, this can be particularly important because we can see inside the character’s thoughts. He or she is telling us the story directly. Is the character honest? Or does the character choose to conceal things? Even an honest character can have mistaken or biased views. How does this affect your character’s voice?
A helpful exercise is to study the voices of various first person narrators. Ideally, you’ll study good writing, not bad. For example, I recommend Lolita by Nabokov.. It’s a scary book, but the voice is ridiculously compelling. There are plenty of great first person books out there.
Here’s a list I found on a Google search. Weirdly the first book on it is Lolita. http://www.amazon.com/Novels-Written-First-Person-Perspective/lm/R2KULH2XG4O05S
It’s a place to start.