The materials you purchase in Redecor cost either gold coins or cash. You can see the price of the material below the material icon. By choosing the material and tapping 'Buy' that appears on the material icon, you can purchase the material, and it will appear to your inventory. If you run low on cash, you can convert your gold coins to cash. Once you've used all your coins and cash, you can buy more gold by tapping the gold icon on top of your screen. You'll also get more gold and cash as a reward by voting, and by submitting your designs. The higher you rank, the bigger the prize!