First App.
Your learning space A little progress, every day
CHAPTER 01Lesson 1 of 36

Meet Codex, your app-building helper

You bring the idea. Codex helps turn it into a working app, one conversation at a time.

On your Mac4 min read & do
By the end of this lesson

Understand which jobs belong to you, Codex and Apple’s tools.

YouThe idea
CodexThe files
XcodeBuild & test
Your iPhoneYour app

Let’s do this

  1. 1
    On your Mac

    Think of Codex as a practical helper

    A normal chat can explain how to make something. Codex can also work with the actual files on your Mac: writing code, changing screens and running checks. You tell it what the app should do in ordinary words.

  2. 2
    On your Mac

    You are in charge of what the app does

    You might say, “When I tap Add, let me write a new reminder.” Codex works out the code. You try the result and say what feels wrong. You do not need to understand every line of code, but you do need to check the app behaves as you intended.

  3. 3
    On your Mac

    Meet the other two tools

    Xcode is Apple’s workshop. It turns the files into an iPhone app and installs it for testing. The simulator is a virtual iPhone on your Mac. Later, you will try the same app on your real iPhone.

  4. 4
    On your Mac

    Start with the ChatGPT account you already have

    ChatGPT Plus includes Codex usage. Sign in with that account when the setup lesson asks you to. The allowance has limits, so work in small steps. You do not need to buy a separate API key for this course.

    See the official Codex setup (opens a new tab)
OpenAI’s setup guide showing the menu for choosing Codex in the ChatGPT desktop app.View larger
Screenshot of OpenAI’s official setup guide, captured 7 September 2026. The guide’s interactive illustration shows where to choose Codex.Screenshot source (opens a new tab)

A message for Codex

I am new to building apps. My app idea is my first iPhone app. It will help the people I want to help to [describe the problem my app should solve]. Explain things in plain British English, one small step at a time. Tell me which steps happen in Codex, Xcode or on my iPhone. Ask me one question at a time when you need to understand my idea. For now, explain how we will work together. Do not create files yet.

Make this message fit your idea.

What you should see

You can say: “I decide what the app should do. Codex helps make it. Xcode lets me run and test it.” You can move on without installing anything yet.

A couple of words, explained

Code
The written instructions that make an app work.
Simulator
A virtual iPhone that runs on your Mac.

If you get stuck

Go at your own pace. There is no test to pass.

Checked 7 September 2026. Screens can change; the official guides below have the latest steps.

Codex setupChatGPT plans and Codex usage