To get your voice ready, find yourself a voice coach. These folks are mostly in the Los Angeles area, but plenty do voice over lessons online and over the telephone. As you work with the voice over coach, you will be taught breathing skills, voice manipulation skills and reading techniques. All of which are essential to sound solid!
Practice. Practice. Practice. Nothing makes a voice sound better than putting it out there. Whether talking to a friend or walking down the street, grasping how to use your voice is very important.
Find yourself some voice over books and read up on what the business is like. The best book out there has to be: The Art of Voice Acting by James Alburger. This gives you the entire spectrum of what the business entails.
Create a demo reel and find yourself an agent. Once you have a grasp on the business and your voice in check, it is time to really get yourself out there. A strong demo will provide your vocal talents to those who make decisions. And to get to those people, you are going to need an agent. This step is the last step as you need to have your voice ready prior to creating a tape or it will sound terrible.