Whats the easiest way to refute young earth creationism?

My little brother has recently become a fundamentalist Christian...I'm an Agnostic, I honestly don't know a lot about science..but i trust what most scientists say more so than a 2,000 year old book..I mean..what are some basic things I can say to him? I mean, evolution is a very complex issue..but I mean if the earth is in fact millions of years Old and not thousands..there has gotta be some overwhelming evidence...what can I start with?