A voiceover demo is the most important tool you'll need to have a chance to succeed in this business. It's your calling card and it's a process that will require some patience.
In my opinion, a demo shouldn't be on your radar until you've had plenty of coaching. Many talents tend to rush the process by skipping coaching to get to the demo production phase quicker. Unless you have plenty of experience, I would strongly discourage doing that. Coaching is highly effective in helping you improve your craft, and think of it as a plethora of rehearsals for your demo! Your demo should be recorded AFTER you've had plenty of coaching.
Demo mills have stressful deadlines, and they're constantly pushing their talents into a studio long before they're ready. For example, some will do a weekend workshop and then push you into a recording studio to record your demo on Monday. Unless you have experience, you will not be ready this quickly. The proper coaching process should take several months which is a good thing. You'll need lots of practice, time and patience.
If you're inexperienced, don't attempt to produce the demo yourself. Self-made demos are a recipe for disaster. Great if you want to impress your friends and siblings but horrible if you're going to impress a talent agent or producer. Call any talent agent or production director in the country, and they will tell you the same thing. Your demo needs to be put together by a professional demo producer with plenty of engineering experience.
It's also vital to have original content on your demo. This criterion should be based on your hobbies, interests, skill set, vocal ability and range. Everyone is different and everyone will have different strengths. But again, the demo process should never be rushed. It's too important of a marketing tool!