How to Use Transmog in the Game
The Ethereals from Outland have set up shop in Azeroth to assist with Transmogrification and Void storage, they seem to have taken on the task of reforging items too.
Firstly you will need the item that you wish to transmogrify, and the item that you want it to look like. You need to own and be able to equip both, so if you haven't already now is the time to read around this site, check out the cool outfits and go run some old content so that you can look just the way you want to!
When you've got your items go to the Transmogrification Shop in your capital city
Horde - Orgrimmar - The Drag
Alliance - Stormwind - Cathedral Square
Remember if you get lost you can always ask a city guard for directions!
Ask to transmogrify gear and a window will open showing the items currently in your gear slots. Open your bags and drag the item onto the slot, a price will appear at the bottom of the window. We haven't got a great guide yet on the cost of Transmogrification and it does seem to be related to the strength of the item however most of my usage so far has been in the realm of 7-20G per item which is quite reasonable for sexy outfits.
My only annoyance so far is the lack of bank and mailbox in the Transmogrification Vendor however as a Maxed out Engineer a Jeeves and a Moll-E work just fine!