a) talk to as many people as you know ( at least 12 people), and ask them whether or not your software idea is a good concept or a bad one. if they all say it is great idea, forget it - there is no market for it. On the other hand if they all raise one eyebrow and give you funny looks and say they can't see the software succeeding - then it's a go. I'll explain it - if anyone here needs it explained. If you need the above explained, then you lack "tradeskills" (intrinsic business skills that you developed before age 16!!). If you don't have "tradeskills" then you better find a partner who does have tradeskill! At the same time, you need a third person to help you develop your partnership --- or you will be very sorry.
b) when the software is more or less finished, you talk to at least 12 "potential users" of the software and have them use the software for at least a week and then fill in a concise evaluation sheet. then collect and analyze the questionnaire.
c) then develop you marketing story/ !