1. Get some training
You wouldn’t start hacking away at people’s hair without attending cosmetology school (I hope). And you can’t start recording voice-over without training. Even if you are an experienced actor, voice-over is a different kettle of fish altogether. So take some classes! A quick Google search will show you what’s near you, or there are plenty of online classes available from anywhere. Once you have some general voice-over training, take note of what you have a proclivity for and then get some specialized training (Corporate videos or medical copy? Audiobooks or animation? Video games or voicemails? They all require different techniques and it takes a few years to get those in your wheelhouse.) DON'T let anyone convince you to record a demo at this point. You're too new; your demo will be sub-par. Also demos can be very expensive. Wait till you get some experience before making a demo.
2. Purchase some equipment
3. Audition, audition, audition
New hairdressers (I am definitely pushing this analogy too far but I’m sticking with it on principle) practice their technique on volunteers and low-cost jobs before they start working full-time in a fancy salon. And you need to get some practice before auditioning for bigger and better roles. To do this, join one or two of the mega-sites for voice-overs and start auditioning. Voices.com, voice123.com, VoiceBunny.com, etcetera. Use your new equipment to record some basic demos. Be honest in your profile. Say you're new, say you're training, say you're looking to acquire experience, and audition for pro-bono and low-budget jobs. The goal here is to get your toe in the water, not to get a Toyota commercial.
4. Rinse & Repeat