G, C, and D Using the Bar Chord
This is the easiest chord on the banjo because it only requires one finger! For the bar chord, take your fretting hand index finger and place it behind any fret on your neck. The lower frets (1-5) may be easier to push down but practice pushing down on the 7th fret to see how it feels. The bar chord on the 5th fret is a C chord and on the 7th it's a D chord.
The Bar Chord Video Tutorial
The tab below gives you a way to practice switching your chords. Play along with it and work on transitioning from your G chord to C and D.