Apple Human Interface Guidelines
Apple provides a number of resources that can help you designing your application and laying out your interface. In some cases these are not just rules and guidelines you must adhere to in order to get your application approved on the app store - they are also an extremely useful tool-kit that can be used to develop a great user experiences on any device, or platform.
The Apple Design resource, is the main link to start learning about iOS application design. Within the guideline section you'll see a list of Human Interface Guidelines for the different Apple products: macOS, iOS, watchOS and tvOS. The most important document for this class is the Human Interface Guidelines for iOS.
This guide outlines all of the rules should be followed when developing applications for devices that use iOS. This includes topics like choosing the size of buttons, how close buttons should be to each other, and what feedback the user should receive in order to conform to Apple's standards. This information has been meticulously researched by Apple and will help you build well structured, easy-to-use applications, regardless of the scope of the project.
You should start to get familiar with Apple's Human Interface Guidelines, weather you intend to develop for Apple devices in the future or not. It does not matter if you work on the code, or the design side this yet another useful resource to add to your toolbox.