First, question yourself. Look at your idea as a tree, and you are a lumberjack. Take an axe to it, and try and knock it down. If it's a great idea, then it will easily withstand the cutting blade. If not, it will fall and you have no need to invest in the idea any further.
If the idea is still "standing," then clearly define who will use the idea and how they will use it. Understand how it will be used by the target audience and how they will use it OVER TIME. If you create a mobile app that is only used "once or twice" then it's an offering that will not be integrated and integral to users lifestyles. You may be happy with casual usage, so this may be enough. I wouldn't be. I'd aim for a mobile app that becomes a "lifestyle" app, one that people use all the time. If you don't have that, consider going back step number one, above.
Once you have confidence in the idea and that it will be one that will be an integral part of people's lives, mock it up. Make a functional prototype, and then test it. Accept that the first version won't be right! Take the feedback to heart: revise, polish, improve. Make it better. Iterate, refine, and focus on making it work for the intended market and users.