How do I write a software product definition?

Question

I would like to learn how to write a software product definition. Therefore I am looking for online materials or books which would help me to learn more about this topic. I would like to learn, for starters:

• what must be included in such a document
• what must not to be included in such a document
• how to make a product definition to sell internally the product
• finding balance between use case descriptions (the why), and feature descriptions (the how).

I am aware that it is not something that can learn in 15 minutes but I think such a discussion could help me to have a good start.