Is there a native QR Code reader on iPhone?
You don’t actually specify whether your iPhone was brand new, or just new to you, but if you’re running iOS 11 you can just open the Camera app and point it at a QR code. It will automatically recognise it for what it is and present you with a notification you can tap on to complete the action. 

For example, using your camera to scan a QR code will prompt you to open a website, make a phone call, add a contact, launch a video, make a payment, etc.

Below are some screenshots showing examples of the prompts you can get:







By tapping the prompt iOS will open the appropriate app and perform the related action.

NOTE: If the iPhone you got for Christmas is not a new iPhone and it's running an older version of iOS, then you’ll need to install a QR reader app (of which there are many) or, if the iPhone supports it, you can upgrade to iOS 11.