When you have found an item there are a couple different ways to bid on it!
Ensure you are logged in
Scroll to the bidding box
The box will be pre-filled with the next price increment. You can continue with the next increment or choose your own amount to bid.
Clicking "Place Bid" will bring up a secondary window to select an optional "Max Bid" - this will be your maximum amount you are willing to pay for an item. Selecting a max bid will keep you on as the high bidder until someone else outbids you.