Its very easy, if a bit confusing to start with. You basically get charged an insertion fee which depends upon how much the starting bid is. So if you are selling something which you want a starting big of £10 this will cost slightly more than a £1 starting price. If you have additional photos or highlighted listing that cost a bit more too. All the costs are expalined as you set up an item to sell.
When the auction is over you then get charged based on how much you sold the item for.
For example I put something on with a starting price of £5 and it sold for £34 which cost me £2.12
You give ebay your bank details and they take it from your account once a month.
You need to give P&P amounts on the listing, go to the Post office web site to get costs and use some kitchen scales to estaimate the postage. Alternatively take the item in loose packing to a pst office and get them to tell you how much it will cost to send.
Don't go over the top with the costs as buyers get upset (understandable) if you send the item with a first class stamp on and charged them £5.
The buyer will pay you for the postage costs so its dosn't really matter. The buyer would probably appreciate first class post as they will get the item quicker.
If you are only selling a few thing then just ask for payment by cheque and make sure you give them time to clear before dispatching goods. Keep the buyer informed and you will be fine.