If they give you a reservation they will probably want a deposit - they'll ask when they've reserved an actual pitch. Some very popular sites will charge a fee to reserve rather than deposit, ie it is not offset against the pitch fee when you arrive. If there's no need to reserve in advance for the date you want, they'll say so.
To pay, credit card is by far the simplest, if their reservation offer includes the letters "C.B." then they will accept any major credit card, just phone them or send the numbers in a letter, not by email, obv.