do you have a capo? If not I would suggest sticking to the key of G for now. Once you get the basics down on that you might choose to learn out of other positions.
The main chords in key of G are G, C and D. this will cover at least half the songs in bluegrass probably. (once in a while there is another chord thrown into a song) If you are wanting to learn a particular song let us know, we probably either already know the chord progression or can figure it out fairly quickly.
The second most used postions that banjo players will play out of are:
C postion includes: C, F, and G positions
D position includes D, G, and A positions
most banjo players spend 90% of their time playing out of G postion.
If you learn a song in G and a singer wants to do it in A you just put a capo on the second fret and tune or capo the 5th string up two frets (to an A note) and play as if the song were in G.
Capo 3rg fret = Key of Bb
capo 4th fret = B
Capo 5th fret = C (or can be played in open C position)
Capo 7th fret = D (not mormally played here) usually played in D position w/ 5th string tuned to A
Key of E is usually played with capo on 2nd fret out of D position
Key of F is usually played with capo on 3rd fret out of D position
hope this answers some questions.