One of the easiest things about playing the five-string banjo is its open tuning. By strumming the banjo with all of the strings open, you play a G chord. Hence, a C chord would be a bar across all of the strings at the fifth fret, a D chord would bar at the seventh, and so on.
The second, third, and fourth strings are tuned the same as a guitar. The first string on a banjo is a D, as opposed to an E on a guitar. You can use your guitar shapes on the banjo, but just be sure to move the note on the first string up a whole step (two frets). Look at the red dots on the chord shapes below.
In this example, there are two G notes to start the phrase. Instead of playing both of those notes on the same string as you would on a guitar, you alternate between the first string at the fifth fret and the open fifth string. This makes it much easier and faster to play than on a guitar.
On the banjo, the next three notes are fingered differently (F, E, D).
The F is fingered up two frets.
The E is played on the second string at the fifth fret.
The D is played on the open first string.
Alternating between strings allows much smoother and faster play when finger-picking.
—adapted from “5-String Banjo for Guitar Players,” by David Brandowski, Deering Banjo, 2010.