Now that you have a good idea of what you want the app to do, who it is for and what it's competing against, you should feel confident enough sharing the idea with others.
Make a list of of all of the people you know who match the intended audience. Go talk to each one and, after asking them to treat it in confidence, bounce your idea off them. Don't just tell them about your idea though, let them read what you have written.
Take good notes about their feedback and try not to get upset when they disagree with your 'features' and 'benefits' or criticise your drawings.
Don't make any changes to the written description until you have talked with everyone on the list, then see how many people commented on the same things.
Make sure to ask them about the price. Is it too high? Too low? Just right?
Try to get a 'buy' signal from them. Most friends won't want to hurt your feelings and may be more positive than they really feel, but if you try to commit them to buy it, they will quickly let you know. Try saying something like "So, if I make the changes you have suggested, you will definitely buy it?"
Once you have analysed the feedback, go ahead and refine the document, sending it to each person you spoke to once again for a quick review.
Make a list of the people you know who match your intended audience.
Talk to each one about your idea and review their feedback.
Convince at least one person in your intended audience to agree to buy your app.