Look at the item you wish to buy to see what the starting price or current bid is. This will give you an idea of what you should bid. If you want to see the entire bidding history of the item you can click on the 'Auction History' tab.
Once you decide what you would like to bid, enter in the number and click 'Bid'. You should see a pop-up on your screen that states that you are now the highest bidder and you will also receive an email.
If you get outbid you'll receive an email alert giving you the chance to increase your bid. You will also receive an email if you're the winning bidder. However, you can only win if your bid is above the reserve price.