Start by buying, and selling low value items to get used to how things work.
Take good photos.
If you accept PayPal you must understand the rules. Send goods only to the address that is registered with PayPal, and send by trackable means. That way you will be protected against fraudsters and scammers.
Weigh the item plus the packaging before you do the listing. You need to know in advance how much to charge for postage and packing. Too little and you will be out of pocket, too much and you will put buyers off.
Start the listing price at the price you are prepared to pay. But understand the fee structure. It costs more to list an item for �1.10 than 99p for example.
Bear in mind all the fees - listing fees can be high if you choose to use all the bells and whistles, final value fees and PayPal fees.
I really recommend you start by selling low value, unwanted items.
Good photos are essential, as is a clear, accurate and truthful description.