Each seller on eBay will specify the methods of payment they accept underneath the current bid price (or, in the case of 'buy it now' items, underneath the fixed price which the item is offered for).
By far the most popular payment method is PayPal. When you set up a PayPal account, you can simply pay for an eBay purchase with your debit or credit card. (PayPal forwards the money to the seller, minus their commission. You pay nothing to use the service as a buyer):
http://www.paypal.co.uk/uk
Chris