Developing the code for an app is just the first step. Design, Marketing, Monetization, ASO, Localization, Accessibility, Customer Support are equally important. Only knowing those makes great apps successful.
I recommend everyone who wants to learn the basics of Swift and iOS programming to go through this course by Paul Hegarty.
Study the "Human Interface Design Guidelines" to learn how to create apps that are ready for a 21st century audience.
Learn how to make your app findable on the App Store. Creating the app is step 1, but you actually have to find the right audience to download your app.
Learn about different ways to monetize your app. Free download with In-App purchases? Subscription-based business model? You have to find the right model for your use-case and you audience.