You need to start getting into conversations with people who are likely to be potential users of the app. Finding related Twitter chats, for example, and getting involved in those chats, asking and answering questions, is a great way to build community and buzz before the product is launched.

This also will help you in your development, as things will come up that will make sense for you to include or exclude from your app, and you will find a great community of beta users as well.

Let me know if you have any further questions or need assistance in this - social community development is a crucial part of any business now, and I speak on the topic often at conferences.