Did the wrong item arrive? Did your order not arrive at all, or was it significantly different than described? Open a dispute - and open up the lines of communication. You and the seller can discuss the problem and try to come to a mutually agreeable resolution.
As many disputes are caused by misunderstandings, they are often easily resolved. However, if you and the seller are unable to come to an agreement on your own, your next step is to escalate it to a claim, which means PayPal will make a decision in favor of either the buyer or the seller.
If you decide to open a dispute, be aware that you must open it within 45 days of the date you made your payment, and that you and the seller will have 20 days to work things out.
And if you choose to escalate your dispute to a claim, you must do so within 20 days of opening the dispute, or it will be closed automatically.