Bluegrass starts with Bill Monroe. He is to bluegrass what Bob Marley is to reggae -- he invented it.
Many contemporary performers -- Allison Krauss, Sam Bush, Bela Fleck -- have deviated from the classic structure of Monroe's and created a subgenre sometimes called "Newgrass." Some really great stuff.
The Del McCoury Band still plays traditional bluegrass, along with many others, and the recording Steve Earle made with them, "The Mountain," is a must. (Everything Steve Earle does is a must, but this is his pure bluegrass album.)
Many of the songs from "The Mountain" were used in the soundtrack for the movie, "You Can Count On Me."
I'd say check out the reviews and find a decent Bill Monroe recording, but definitely -- "The Mountain."