Under Petz on the menu bar is "Item Shops"
This is what you use money for. You can buy stat increasers, items to increase your pet's pleasure and happiness, and some other items that are at the shop. The other shop is where you buy food for the pet's hunger level.
Next is the arena. It's where you make challenges and fight. You can make wagers on the battle (it's all easy, they don't hide anything so it won't be hard to find)
Next under Petz is Bank. I suggest highly you open up an account, click on Bank. Deposit your money and each day you get 21% interest (a day). But if you visit the bank then you won't get interest on the day you visit it.
You see, you get money randomly sometimes. To tell if you do or not just find a thread you've posted it an check the amount of money it shows.
Adoption Center is just if you want to disown a pet.
Store. You spend money here, too but not for Petz. It's for yourself, like glow to the username, italic to username, etc.
Under that is Information. I suggest reading that.
Stats shows the strongest petz and richest users, maybe other things.
When you buy things, it goes to your inventory. To use the items click on Petz, then Inventory and it will show the item. Then click 'use' to use it.