This step is very easy but it takes a while. Open up the Mac App Store, search for Xcode, and hit install (or update if you have it already). If you're not able to update because your operating system is out of date, we recommend updating your operating system to the latest version and then updating Xcode. You may run into issues further down the line if your Xcode version is out of date, for example you may not be able to submit your app to the App Store.
Open Xcode, then choose "Preferences..." from the Xcode menu (or press ⌘+,). Go to the Locations panel and install the tools by selecting the most recent version in the Command Line Tools dropdown.