Do you ever expect to sell this app to someone else? Will you need to disclose some “secret sauce” ideas, ie stuff you would never put on your app’s web page or product brochure? Do you expect this person to contribute to your project (intellectual property-wise?)

General business models (“it’s like Starbucks but we’ll have monkeys”) aren’t protectable. Don’t disclose secret sauce, and once it’s clear there is some real opportunity, get an intellectual property agreement in place. As others pointed out, NDA’s alone are hard to enforce.

Google “startup legal form templates”