If you are on the page that shows all of the items, then you'll need to click on one item of interest at a time in order to place a bid. You can either click on the item's image or you can click on the item's Details button to proceed to the details page for the item. Once on the details page, you should see a button that says Place Bid (if you are signed in), Sign in to Bid (if you haven't signed in yet), or Add a Card to Bid (if you still need to add a credit card to your account). Enter the amount you want to bid (this can also be a max amount if you wish to let our system bid on your behalf up to your Max Bid) and click on Place Bid to continue. A window will pop up asking for confirmation of the bid to proceed, then a new window will pop up where you'll need to select a credit card, shipping options, and agree to terms before clicking on Place Bid one last time. You will then see an alert that you are either Winning or Losing. If you don't see one of these notifications, then you missed a step in the bidding process and will need to repeat, being sure to complete each of the required fields in the pop up window.
It is also a good idea to read the Description and the Shipping Info before placing a bid so that you are aware of any restrictions before committing to placing a bid.
If you are on the Details page of the item and you see none of the aforementioned bid buttons, then that means the auction has not yet opened. You'll need to wait until the auction opens for bidding to see the bid buttons.