Elevator Pitch
How would you describe your app to someone if the only time you had was the length of an elevator ride? That's your app's elevator pitch, a brief summary of who your app is for, and why it's useful to them.
Come up with ideas
Start with the big questions. Why should people use your app? What do they really need or want to do? What makes it different from other existing apps? How will people use it?
What is the purpose of your app?
If you already have a list of tasks, an idea for your app and the type of user you are targeting, it’s time to create a goal statement: Define your app’s purpose and the most important user in one sentence. This is your main use case.
Example: A wish list creation tool for people who never do impulse shopping.
Focus on one main use case
It is possible that you cannot include all the tasks on your list in your goal statement. That is ok, because you should focus on one main use cases, which is how most people will use your application. Awesome apps do one thing well.
If you are not sure about the difference between a web app and a website, this checklist can help:
http://www.html5rocks.com/webappfieldguide/know-your-apps/app-checklist/
Concentrate on a few key features
Look at your task list again. Filter your list through the goal statement. If the tasks do not align with your goal statement, exclude them in your app.
Describe each core task as a feature and then ask yourself, is this feature essential? Or is it nice-to-have but not required by the target user to complete the defined task? Be honest with yourself. If you end up with a short list of features, you are on the right track.
Remember, the best apps usually do one thing well. Apps often fail not because they have too little features, but too many.
How will people use your app?
By now, you’ve identified your main use case, target users, and key features. Your main scenario should also consider the user environment in which your app is used. For example, a young mom with her baby at daycare might use your app to note a nice stroller (potential multi-tasking, pausing and continuing the task later). A different user might plan her next laptop purchase at home, in an armchair, without interruptions.
Your app in the most basic form
Once you have a few key moments, you can translate those steps into screens. You can sketch out the user flow, that is, what do your users go from one screen to another to complete a task.
Think about the information a user wants to see and record. Consider which user interface (UI) elements correspond to the information above. Put things that your users need most often in the most prominent places in the screens. Even just writing out the sequence of moments helps define a wireframe model of your app and its features.