Three easy banjo chords are G major, C major and D7. With a standard banjo, hold the banjo's neck with the left hand, and use the fingers to press different strings against the fret board. Use the right hand and fingers to strum.
Banjos generally use open-G tuning, which means that you can play a G major chord by simply strumming the banjo without pushing any strings against the fret board. Play C major by strumming with the index finger pushing the second string from the bottom against the first fret and the middle finger pushing the fourth string against the second fret. Play the D7 chord with the index finger in the same place but with the middle finger pushing the third string against the second fret.