I'm looking to book a trip in September with some friends, NY - LAS

I was looking into rewards cards to take a chunk out of the airfare costs -- most airlines doing these spend $xxx get xx,xxx miles programs, do I have to wait to book flights until I get these miles? Or can I apply them retroactively? I know Capital One cards allow you to apply miles earned to previous purchases, I'm not sure about all these airline cards though.

If anyone has any experiences with these or recommended cards it'd be greatly appreciated.

Don't know an exact credit score, but it was good enough to get me a 0% new car loan through chase 2 years ago.