For voiceovers I strongly recommend private coaching to get started on the right foot. I recommend well-known voiceover performer Joan Bogden. You can read about her and contact her through Voiceover Coaching
It is necessary to put together a professional reel, and that can be pricey. However, it's worth the extra expense to have a professional put it together for you. This is indeed a very competitive area of the business, but it can be extremely lucrative.