Assuming you have actually identified someone in this role and you're asking about the legal side of things, I'd say that beyond a legal entity by which to enter into an employment or contractual agreement that has the standard assignment of rights, confidentiality, etc, you're probably fine for now. Depending on where you're operating from and whether you actually have an actual office or not might impact your need for a business license.
But if you're asking how you would actually identify this person, that's a much different answer. The reality of it is that unless you have already raised or can self-fund a full-time salary of an iOS developer (which will range $85,000- $140,000 for just salary anywhere in North America depending on experience and location), you're better off looking at a mobile development agency.
Also, many great iOS developers lack strong back-end skills, so in order to publish an app, you're likely looking for two engineers, not one. Those that pass themselves off as "full stack" are usually marginal at both. So either you have to be able to show that you have raised or can self-fund two people's salaries for a year, or you won't be able to attract full-time talent. Also, I haven't mentioned anything about design, which depending on the nature of the app, would require a third person at least on contract to design and iterate the user experience.
In most cases, non-technical founders that want a product built should find a reputable mobile agency to build the first version of their app. I know of several and am happy to provide further information, if you're serious about building your app. Beyond the cost of building a single platform app with a contractor (I would budget $50k at a minimum), you should also be sure you have at least $50k in customer acquisition budget comfortably allocated and that's just to take a "realistic shot" at having an app get any traction.
So if you can't raise or spend USD $100,000, I would say that as a non technical founder, you're better off pursuing a different line of business. That's just the reality of the mobile app marketplace today.
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.
Granted we have no idea of what you're looking to have a developer build, 100k or hiring a full-time iOS dev AND full-time backend dev is certainly not necessary. There are plenty of great freelance developers and leverage frameworks like Parse for backend make it extremely easy to deliver a "full" app. I've helped many entrepreneurs precisely with this process. Let me know if you want to further discuss.