Place the item in your room and double-click it. Click "Redeem" to change it back into credits.
Gold bars cost (and are worth) 50 Credits.
Money bags cost (and are worth) 20 Credits.
Gold coins cost (and are worth) 10 Credits.
Silver coins cost (and are worth) 5 Credits.
Bronze coins cost (and are worth) 1 Credit.
Go to the Habbo catalogue when signed into Habbo, and you can purchase the items there.
You can find the Habbo Exchange in the Habbo catalogue. You can access the catalogue when logged into Habbo through the button on the bottom of the window
The Habbo Exchange is where you can turn your Credits into special Credit furni, which you can then trade with other Habbos.