When you think of a banjo, you might be thinking of melodies (like “Dueling Banjos”) but that takes years to learn, and LOTS of banjo players don’t play melodies (they play chords, and then sing the melody). If you just want to play chords, just place your finger across one of the frets, then strum the strings. Boom...you’ve played a chord. Now put your finger across the strings again, but at a different fret, then strum it again. Boom...you’re playing another chord. I recommend labeling each fret with masking tape and marker, so the chords will be easy to remember.
Note: If you just start picking the strings (or even strum them), that’s a G chord. From the nut (the top of the banjo), the first fret is Ab, the second fret is A, then Bb. C, Db, E, F, Gb, G. You’ll have more frets, but they are really high notes, and you’ll probably never play them (just ignore those super-high frets).
Note: Don’t play your super-short G string (that’s the one that is missing if you have a 4-string banjo).