The Apple App Store is an application that installs other applications.
You can find App Store in the Applications folder
Search for 'XCode' and click the 'Install' button
Open the Preferences menu and select the Downloads tab. Then click the 'Install' button next to 'Command Line Tools'.
Open the Preferences menu and select the Downloads tab.
Then click the 'Install' button next to 'Command Line Tools'.
Go on to OS X RVM