There are whole books written on the topic. Entrepreneurship programs cover a lot of techniques.

What I usually do is analysis: SWOT: Strength, Weaknesses, Opportunities, Threats. In other words, why am I in position of doing it better, why other can do it better, is there an opportunity that only I see, are there threats from other places, like other industries, government, etc.

I search for a problem to solve. You need a problem that you clearly see that people are struggling. That is why experience at some area is very important. For example, working in the kitchen at a restaurant, you will see more problems and inconveniences, than me who only have ideas. Working on that place, you know the people personally, you have your PERSONAS, you first testers!

Once you have a problem, once you have a good analysis of what's possible, where you are in the market, then comes the real requirements gathering, and subsequent. Don't hurry, talk to people gain more information, read more books on the topic, meet people.