One of the most-frequently asked questions I get about starting a voice over business is, “How did you learn that?” There is no secret sauce. I learned how to do voice over the same way that you and I learned how to talk: trial and error. Go find a large collection of vocal recordings, like librivox.org, and start listening! You’ll hone your ear for what kinds of vocal techniques sound good in different situations, and you’ll also learn about how to make a good recording. An obvious but real example is: you don’t want to sound like a sports announcer when you’re reading a novel. You want your voice to match your context.
How do you learn context? Trial and error, and it helps to find somebody who is willing to be your sound board. That way, you can tune your voice to different contexts with less fumbling and better focus. Volunteer for a local radio station, if you have one in your town, or join an improv acting group. Groups like that will help you learn vocal delivery much faster than just talking to yourself, although that can work too.