The Transmogrification vendor for the Horde is located in the Drag, across from the quest giver for the cooking daily. For the Alliance, he is located on the outer ring of Cathedral Square.
Talk to the vendor to start the Transmogrification process.
This is actually an incredibly easy process. Simply drag the item in your bags that you would like your piece of gear to look like to the appropriate slot on the paper doll. The final cost of the transmogrification will show in the bottom of the window. A purple glow around the icons means you haven't finished transmogrifying yet.
Once you have completed the process, the lines around your gear will no longer glow and the gold total disappears. All done! If you want to transmogrify your gear to something else, just drag and drop onto the paper doll again.
Your gear will still show with the appropriate icons on your character sheet; however, the item you used to transmogrify is shown in purple text under the real item's name. Easy-peasy!
Void Storage is also an easy process, but it's expensive. Remember that this is the PTR, and costs may change -- but consider this your gold sink. The cost to open your void storage vault is 1,000g.
Once you've opened your vault, you'll get a mini-tutorial on the process. Place items in the upper window to put them in your bank. There is a 100g charge per item at this time.
To remove items from Void Storage and place them in your bags, drag the items to the lower half of the window. As with deposits, there is currently a 100g per item charge to withdraw items.
You must hit the transfer button before making any transactions -- and from the looks of it, you can withdraw and deposit up to nine items at the same time.
Your gear must be repaired before it can be placed in void storage; it won't let you put it in the bank otherwise! If you are placing gear with enchants and gems in the vault, a confirmation window will pop up letting you know that all gems and enchants will be removed.
Unique items cannot be placed in Void Storage. This includes all legendaries. However, with the amount of bank space opened up with this process, having to hang onto your legendaries isn't a problem!