How do you choose a target audience?

How do you choose a target audience?
1. Decide what you are Selling or Promoting
2. Spend time thinking about and searching (on and offline) for different groups of people/consumers/readers that may have direct or indirect interest in what you are trying to market.

For example, if you are selling a book about sacred healing remedies;
You could consider the following Markets as targetable
Reading/Books (wide audience high competition of other websites or businesses)
Herbs related (scientific and mystical, consumer and product suppliers)
Medical and Holistic Niches
Health communities, mystic communities, reading communities, biological plant niche...
Gardening Niche
Occultism, Spiritualism

You then need to check how many businesses/websites etc are competing and how many consumers available or reachable by you in that niche

its a never ending science and you need truly to intuit and understand it from 'feel' because the permutations are endless. it is the ability to see through the holes in markets and choose one that is not already over exploited, and also the ability to know how to stimulate reaction from your customers.. its experience which makes the difference. Trial and error.