There are millions of items for sale on auctions at any given moment, and the breadth of what's on the auction block boggles the mind.
If you know what you're looking for, the best way to find it is to type the item name in the search box you'll see on auction site's front page. If you don't know specifically what you want to buy, but know the general category, use the category listings.
Before you can bid on anything, you need to register with an auction site such as eBay Auctions. It's a simple process that requires you to give up a little personal information, but there's nothing that says you can't lie about yourself if you want to (you need to provide your real email address to respond to the confirmation letter the auction site will send you once you have completed the registration form).