First of all, if you're really new to the site and are curious about making money on it—maybe you've heard a lot about it, or you have a friend who's made some money with it before—start by just getting to know the site as a buyer. Find auctions for products you're interested in buying and place some bids to see what the process is like from the perspective of a consumer. Ask yourself why you chose one product to bid on over another, why you bid the amount that you did, and why you stopped bidding at a certain price point. How was the product presented? What did the seller do to make it more appealing to you?
Once you're comfortable with the eBay experience as a buyer and understand how the site works, try selling something. But start small to get the hang of being a seller. Find something around the house that you don't need or want anymore, that you can use to get your feet wet, but aren't expecting to make any real money from. In other words, try selling an old tennis racquet that's collecting dust before selling off your rare stamp collection. Get a feel for how the auctions work as a seller—how long it takes to post the product you're selling, how many people view it, how long it takes before bids start coming in, and whether you are happy with the bids you're getting. Also, figure out how much of a pain it is to package the product up and ship it. For successful eBay sellers, all of these factors are part of the mix, so you need to be comfortable with all of it before you begin to sell anything of value.