A demo recording of you doing voice-over work is your CV and your business card combined. If you're applying for a voice-over job, you can send the potential customer your demo via the Internet. Ideally you have multiple demos for different types of gigs. For commercials, clients want to hear 60 to 90 seconds of voice work. If you're auditioning as an audiobook reader, five minutes of demo proves that you can stay in character over a longer stretch of time.
You will never have to answer to a “boss”, but you will need to be your own boss and manage yourself. You will work on your own a lot of the time. So you need to embrace the benefits and the challenges this presents. Take courses in areas where you need to develop your skills. When you adequately prepare for running a business, you will succeed in its management, even if part of managing your business means hiring out, in areas that you know are not your strongest.
Next, go back to the beginning and start editing out your mistakes. I also like to edit out any abnormally long silences between sentences or statements and any weird sounds that don’t belong. Remember, though, that pauses are ok (and even necessary) to help break up the audio and make it feel more natural and conversational, so don’t go hog wild with it.