How to book a flight online?

I need to know what is the best way to book (not purchase) a flight from London to Denver (USA) online. It seems like that you have to buy the ticket on most websites that I have checked but my plans might change and I might have to choose a different date. I don’t want to be in a situation that there is no place available.

Buy a refundable ticket. It’s going to cost you a lot more.