Ooh, you should go for it! May not happen again in our lifetime lol!
You just select the ticket price that suits your budget. If you select an upper limit it gives you more choice, but you'd have to be prepared to pay the highest price

That's why I just went for the £20 tickets, but there are five of us going, and hey, even if we don't see much, it's the experience right?!
They hold your credit card details until June when they select who gets tickets. If you're successful then they debit your card and if not, well, nothing happens I guess.