It has taken 28 years of hard work and marketing to get to this stage in my career and, quite
simply, it would take hours to begin to explain everything you would need to know about showreels, promoting yourself,
marketing, vocal and studio technique. I could not possibly respond in an email and would have no time for work if i were
to answer fully the number of requests I receive for advice. In common with most of my
colleagues, I am far too busy marketing or working to offer help to anyone who might become
a competitor in one of the most competitive professions in this world.
Over the years I have listened to many recordings of people who have been told by friends that they have a "great voice" and should become a voice-over. At least 90% of these would have been rejected by a producer. Some even declare that they want to "voice" as a hobby and not give up the day job (you can imagine how that one goes down).
If you are trying to find an organisation that trains voice-overs for commercial work - I do not know of any, either in the UK or the USA. There are occasional visits by US professionals who teach two or three day courses (e.g. Marice Tobias) but they are infrequent and not cheap! If you are already a performer and a member of British Actors Equity, the union may run courses in London.
In order to begin and survive as a voice-over today, you need a great many skills - including audio engineering and IT skills, let alone the requisite voice skills. Increasing amounts of audio are electronically commissioned, recorded and delivered. You would need to invest a considerable amount of money in a "Home Studio" - a dedicated work space which is acoustically superb. An investment of at least £7000 would be in order to adequately equip that studio - then you have to have the technical knowhow to work it.
The voice-over business is a cut-throat one. Your first job for an employer is your last if your technical or vocal ability is not up to the mark. Your showreel has to contain material which you can produce on microphone at the drop of a hat: it should contain material which has been broadcast or used professionally (or sounds like it has).
If you are in the UK and live in London, you would be competing with an already oversized population of voices. If you live outside London, you could not survive without a Home Studio. If you live outside the UK then i am not qualified to advise you in any case. Only you can decide whether the investment or risk is worth it.
I am not an agent for voices and when I do need to produce or employ other voices, I already have a pool of talent on which to call.
All this is purely my opinion and I hope it helps. My comments are supposed to be constructive and not dismissive or offensive in any way.