Each site will have a free registration, which you will have to fill out in order to bid on any auctions. There will at least thousands, usually millions of auctions running simultaneously. Each auction will post a desciption of the item up for bid with perhaps a picture, the close of the auction, a display of the minimum bid required for the item to sell, and a listing of the highest current bid. Most online auctions will give you some indication of exactly how much time is left in the sale. The rest of it works like a normal auction.
Sale goes to the highest bidder.
That's the gist of it. The best way to learn is to get out there and try. Best of luck and happy shopping.