Earl Scruggs pioneered bluegrass banjo playing through his use of 3 finger roll patterns in his picking hand (his right hand). These roll patterns gave a consistent drive to the music Bill Monroe was playing and helped define the bluegrass sound. When learning how to play bluegrass banjo, mastering these roll patterns Earl Scruggs used is essential.
There are a number of different 3 finger roll patterns for the banjo. When first starting how to play, the following roll pattern is the one I would suggest you beginning with. This pattern is called the "Alternating Roll" or sometimes it is called the "In and Out Roll". Because it is made up of 4 notes, and you will most often be playing in 4/4 time (4 beats to a measure), this 4 note pattern is easy to loop and fit squarely into each measure.
You can begin playing this roll by playing the 3rd string with your thumb, then play the 2nd string with your index finger, next play the 5th string with your thumb, and lastly play the 1st string with your middle finger. Loop this roll and play it over and over.
Remember to practice slowly, focusing on playing each note cleanly. Use a metronome and focus on your timing and tone. Slowly speed up the metronome as you feel comfortable. Good luck!