This may be a stupid question, but...

I would recommend starting with making an app that just says, "Hello World"!
It was really easy in UIKit, but it's even easier with SwiftUI.

1. Download Xcode

2. Wait an hour for it to download

3. Open Xcode and the Welcome Screen will pop up. Click "Create a new Xcode project"

4. Choose "Single View App"

5. Enter the "Product Name" (name of your app). For "Organization Name", enter your name. For "Organization Identifier", it's formatted in reverse domain name notation, which is basically "com" followed by your domain name, and then your product name. It's ok if you don't own a domain, just make one up. Example: com.myfirstappdomain.FirstApp. Also, make sure "SwiftUI" is selected for the User Interface.

6. Press "Next" and choose the location where you want to save your app. Don't worry about Source Control for now.

7. Press "Create"!

Once Xcode finishes generating the project, you should see the following lines of code inside the editor:

Code Block language
/*
// ContentView.swift
// FirstApp
//
// Created by Your Name on 6/19/20.
// Copyright © 2020 Your Name. All rights reserved.
*/
import SwiftUI
struct ContentView: View {
  var body: some View {
    Text("Hello, World!")
  }
}
struct ContentView_Previews: PreviewProvider {
  static var previews: some View {
    ContentView()
  }
}

Here's the above code explained:
First, you import SwiftUI, which loads the framework. SwiftUI lets you make apps by writing out what you want to see.
To write out the user interface, your code goes in a struct, which is building block that you can add properties and store values in. body is a property, and inside is a Text object (which comes from SwiftUI). What the code says is, Here's a struct that contains some text that says "Hello World!"

In our case, we want to see "Hello World!" (no comma) so you can remove the comma that Xcode auto-generated. Or you can keep it if you want 😁.

Underneath all the code is another struct, ContentView_Previews. This is for Xcode to know what you want to display in the live preview. You don't need to worry about this.

Now all that's left is to run the app! In the upper-left corner of the screen, first make sure you select an iOS device to simulate on (<Your App's Name> → iPhone 8). Then, press Command + r to run! You may be asked to sign in to your Apple ID, and if there's any errors, try signing in.

A simulated device will appear on your screen after a while, and it says, "Hello World"! Here's a screenshot. That's it! Your first app!

To get more advanced, you can make use of resources on the internet. Hacking With Swift and Ray Wenderlich are both free resources that are great places to start (search them on Google, I couldn't put the links because they weren't allowed on the Forums). You can also go on Udemy and buy a course, like what I did (usually around $10) -- I would recommend following more tutorials before you start making your own apps (it'll be much easier once you know the basics). And if you're ever stuck, you can post questions on Stack Overflow, or do what you just did, ask on the Forums!

Happy coding!

Projects are created in Xcode, a macOS app which only runs on Macs.
My advice is to search online for simple app tutorials which Interest you and follow them to learn. As you grow, take apart open sourced Xcode projects to see how they work.
if you have an iPad, Swift Playgrounds is an excellent way to learn the Swift language.
Above all, keep safe...

Try starting here: https://developer.apple.com/tutorials/swiftui/tutorials

You can always check out the free content from hacking with swift too, to get a good start on learning how to make iOS projects using Xcode.

Also you can pass the Stanford course in the "iTunes U" app:

1. Download the app on your iPhone

2. Find "Developing iOS 11 Apps with Swift"

3. Add it to your courses

Btw, you can find the lectures on YouTube.

P.S. sorry for possible mistakes. I'm not a native speaker = )

The 2020 Stanford iOS courses were just released for free on Youtube.

Stanford CS193p - Lecture 1: Course Logistics and Introduction to SwiftUI

Professor Paul Hegarty is a legend and has been teaching this specific course for a decade.

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Hacking With Swift by Paul Hudson

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Ray Wenderlich Tutorials

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If you are just starting, I suggest you avoid all the betas. Stick with the stable version of Xcode and don't worry if you see a million buttons and settings. You will learn them over time.

Find a mentor, be humble and remain enthusiastic.

Welcome to the family!

I want to start making my app, too but should I use SwiftUI or UIKit? What is the advantages and disadvantages of SwiftUI?

@Yagizzhan: SwiftUI is the future but it’s still half baked. That may change in a day or two 🙂

Even so, I’d recommend SwiftUI since it’s not too difficult to use UIKit views in places where SwiftUI has no support yet.

My suggestion is to get an iOS book such as Hacking With Swift iOS by Paul Hudson or App Development with Swift by Apple or any other (those are just my personal recommendations) and read it. You will be introduced with all the steps required to start creating app. I've started learning myself App Development year ago and these books helped me to start understand everything step by step. It's not easy in the beginning but things will come together after some time. I probably wouldn't recommend start learning from Youtube videos or Udemy courses, these can be taken after you've gained some initial skills.
Good luck with your journey!

I agree with @Sangsom about the books. You can even check some out from a public library -- they might be outdated, but anything above Swift 3 will still make sense.

However, books aren't for everyone, and a video course may be a better place to start (you can follow step by step easier). Books can also be expensive, as they average around $50.

The best to start is probably using Apple Book:
« Intro to App Development with Swift »

The App Development with Swift book by Apple is an awesome coding course. However, it's somewhat outdated. The current book is updated from a previous iOS 11, Xcode 9 edition, to a Xcode 10 / iOS 12 edition. While the vast majority is still valid nowadays, in some places it has deprecated code, and it definitely does not contain any of the evolvements in Swift and iOS since then.
Instead, I would recommend taking a look at https://developer.apple.com/tutorials/swiftui/tutorials, as mentioned by Zorro earlier.

Xcode is of course the answer.
I'm also a beginner developer like you.
Learn Swift and create wonders...