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.