Train as an actor, and learn the basic techniques of the acting profession, some of which can translate into voice acting roles. Train by acting in school or community theater productions, or attend classes in acting. Attaining a degree is performance arts is also helpful.
Record yourself in challenging voice roles, and make a demo tape of your best work. Submit your demo tape for potential voice acting jobs.
Use radio to get some voice experience, or use it as a demo reel. Get a job as a deejay, or offer to volunteer your services in order to get some experience and a demo.
Relocate to one of the cities that has a large market for voice acting roles. Move to New York or Los Angeles for the most opportunities.
Attend every audition, even for roles that aren't interesting or don't pay well. Keep going to stay in the casting directors' minds, who might call you back for a different project.
Hire an agent, and have her set you up with the best auditions she can. Choose an agent with experience with voice actors.