Security between .NET MVC and WEB API

We are starting a project which will consist in:

• Web project (ASP.NET MVC)
• IOS app

and both will consume data from a .NET WEB API service.

The WEB API service will expose a POST Method with the url "user/create". But i don't know how can i avoid another apps for making post to this url? I know i need a security protocol, but i wanted to know which one you recommend me, and if you have, an article where is it explained.

Thanks

via Chebli Mohamed