Grats on your new toy.
As far as bags go I strongly recommend Tom Bihn. As his motto says "NO CHEAP STUFF" but they are amazingly well made and last literally for a decade or more. You buy the bag and a separate sleeve sized for your specific computer. He has both backpacks and messenger style bags.
http://www.tombihn.com/
Another Tom Bihn fan. So far it's carried my 17" PowerBook on five continents. I like the backpack so much that I use it for serious day hikes without the laptop sleeve (aka "brain bag").
That said, bag technology isn't Mac-specific. You'll find lots of info about choices and preferences in other threads here. For once, the stuff Windows users post pretty much all applies!