When it comes to writing there are two kinds of “voice”: the author's voice and character voice.
Trying to teach someone how to write Author's voice is like trying to teach someone how to hold onto a slippery amphibian–it's a hard concept to grab onto. Author's voice is the feel of a book, what make's it distinctively one writer's story over another. It's the choice of words and flow; it's the cadence and style. The elements of voice are hard to pinpoint. Think Danielle Steele, Stephen King, John Grisham—all great writers, but completely different in style.
You know when you pick up a James Patterson book, just what kind of book to expect. Not only because of branding, but, because of how he writes.
How do you find your voice? By writing. I remember the moment when I realized, Hey, This is my voice! It happened while writing CLOCKWISE (young adult time travel as Elle Strauss). Everything I wrote before that lacked a certain something. Mostly I was copying the style of other writers that I admired. My work sounded a lot like whatever author I'd been reading at the time. Then suddenly, my work started sounding like me.
This doesn't mean a writer can only write in one genre. Once you've found your voice it'll follow you, no matter the genre.
Character voice is something different. We're talking about what each and every one of your characters talk and act like. Beginning writers often default to the character voice they know best, their own. They have a very strong tendency to give all their characters their own personality. They all say whatever is said, the way the author herself would say it. Do you know what I mean? Have you done this?
So when writing character voice, you need to know your characters a little bit before starting. Do they have a chip on their shoulder? Are they depressed? Are they in love? Are they fighters? Or givers? Do they lack confidence? Or are they overconfident? You need to know something that will shape how your characters speak. See the section on on Felt Need for more on this.
What I sometimes do in second to final drafts is go through each main and secondary characters conversations from start to finish, by putting their name in FIND. You should be able to identify each speaker by how they talk, and not just by the dialogue tag.