Let's start out with the three chords you're probably going to play most as a bluegrass guitarist -- G, C and D. Work on holding the pick, striking the bass strings, strumming the chords, and (very important) keeping an even spacing between picks and strums. This is rhythm guitar, so keeping steady timing is essential. Use a metronome or drum looper to help, and when you get up to speed, play along with your favorite bluegrass songs.
As I explain, if you use a capo (which most bluegrass guitarists do), the shapes will remain the same, but the actual chords will change. This is difficult to explain, but becomes more intuitive with practice.