I've recently bought a rMBP. I've heard I could ask for an OS update for free. Im not sure what that means as I'm new to the Mac. What should I do?
Apple typically calls their upgrade program up to date.
In this case, you would go to this page below and follow the instructions based on the purchase date of your Mac.
Your seller can assist as well - but most people can find things themselves once they know where to look. You can then use one Apple ID on the Mac App store to redeem the Mountain Lion upgrade just like you may have to redeem the iLife (iPhoto, iMovie, GarageBand).
Just go to the Up-to-Date Program Website and request one licence.
You will receive a key with which you can "buy" Moutain Lion in the App Store for free.
There is one more detail to add to the link provided in bmike and Christian's answers. Between entering your request and receiving a key, someone must check that you are eligible. Somewhat annoyingly, this is the case even if you have purchased directly from Apple's online store (in which case confirming that you are eligible based on your order number and/or your serial number could be easily automated).