Everyone can bid on items in other models' wardrobes. When you find the item you'd like to bid on, click "Bid".
When you bid, the price of the item is deducted from you straight away. This is to make sure no one will bid on items they can't afford.
If someone overbids you, you'll get your bid back instantly. You'll also get a notification so you know you can bid again if you still want to fight for that item.
You can see an overview of your bids on My bids page. There you can also raise your bid if you've been overbid. The price goes up 10% every time someone places a bid.
If you win an auction, the item is yours straight away. You have already paid for it when you placed a bid. You'll also get a goMessage notifying about your success.
NB! If any item reaches the price of 99,999 goMoney or Diamonds, it can't be overbid. That's the maximum price allowed on goSupermodel.
Keep an eye out on the Auctions forum. That's where models advertise their auctions.