The first thing you need before you can bid is an eBay account. You can register your username and password Here. Once you have an account and have Signed In you can start bidding on items. Placing a bid is a farley simple process, just click on the auction that you want to place a bid on, select the "Place Bid" button, enter your " Maximum Bid" and then select the "Confirm Bid" button and your bid will be placed. As long as someone doesn't out bid your maximum amount you will win the item.
More Tips can be found in eBay's Help section.