Bluegrass is acoustic guitar with a flatpick: fast, hit hard, and unamplified. The Martin dreadnought above was built for exactly this, a big body that holds its own next to a banjo, a mandolin and a fiddle that are all louder than you are.
What you learn
Flatpicking. Playing the melody with a pick, up to tempo and with an attack that carries. It is the heart of the style and also the hardest part: both hands have to land together to the hundredth of a second, or it sounds sloppy rather than fast.
Boom-chuck. The backup: bass note on the beat, chord on the offbeat. Sounds simple, is not. This is the difference between a band that drives and a band that drags.
G-runs and crosspicking. The signature licks that close a phrase and hand it to the next player, plus the broken-chord technique of Clarence White and George Shuffler.
The repertoire. Fiddle tunes such as Blackberry Blossom, Salt Creek, Whiskey Before Breakfast and Red Haired Boy, and the standards that come round at every jam.
Which guitar
A dreadnought with a solid spruce top gives the volume and the bass this music asks for. Beyond that: medium strings and a stiff pick, because you play with force here. You can start on the steel-string you already own.
Worth listening to
The founders
Doc Watson, Clarence White, Norman Blake, Dan Crary, Tony Rice, Charles Sawtelle, Russ Barenberg
Setting the standard
Bryan Sutton, David Grier, Kenny Smith, Chris Eldridge, Grant Gordy, Courtney Hartman, Jordan Tice, Beppe Gambetta, Larry Keel, Michael Daves
The younger players
Molly Tuttle, Billy Strings, Trey Hensley, Cody Kilby, Jake Workman, Presley Barker, Chris Luquette
Bluegrass and country run into each other; if you want to head that way, have a look at country as well.