When to choose a Product/Service first:
This is a great site to come up with ideas for products and services: BrainStorming.
I’ll make a summary for you:
Solve Problems with Solutions: Think in activities that are hard or a pain to do. Think in a product/Service that can improve that experience.
Market-First: Think in the market you want to approach, research it, understand it.
Personal Strengths: Think in what you are good at.
Importation: Bring/Sell products/services not in your local market, but that exist somewhere else.
Lateral Thinking: Think out of the box, see things with new perspective and create products/services that are innovating, or can use for something different.