If you are happy to pay the buy-it-now price then click the buy-it-now box and you will be taken to the next page to arrange payment.
If you think the buy-it-now price is excessive you can place an auction bid at a price lower than the buy-it-now price. When you then click to the next page the buy-it-now option disappears and the item is open for auction - which can mean, if others are interested, that the final selling price may well exceed the original buy-it-now price. Buy it now also means that you can get the item straight away rather than waiting to the end of the auction time.
Do not complete both the auction and the buy-it-now procedures - one or the other.