If you already know of a developer and you trust them, a verbal agreement might suffice.

If you want to be more "formal" (and protect yourself in case things get a little hairy), then you might want to consider forming an LLC or other type of organization and having a formal agreement in place with your developer.

If you are looking for iOS developers (Personally, I am an Android Developer), give me a call. I know some rather skilled ones (and designers) which I could recommend to you.