Finger picking is the technique of playing the guitar with your individual fingers on your strumming hand, as opposed to using a flat pick. By using individual fingers rather than the whole hand as a single unit, you are able to play several musical elements (bass, melody, percussion) simultaneously.
In fingerstyle guitar, each digit is assigned a number (or letter), and they are:
Your thumb will usually pick the E, A and D strings while your other fingers play the G, B and E strings. In finger style guitar, the three bottom strings (E, A and D) are referred to as the “bass strings,” and the top three strings (G, Band E) are called the “treble strings.” Pinkies are often too weak and too short to finger pick, so they are usually not used unless you are physically able.
To get accustomed to your fingers picking the right strings at the right time, you must learn rolls. Rolls are repetitive patterns of notes that your fingers pick in a certain order using different strings. Playing these repetitive patterns is the only way to master finger picking, and eventually you will be able play anything you want outside of any specific pattern.
The first roll pattern to learn is the front roll. Focusing on the strumming hand, a front roll is simply hitting one of the 3 bass strings with the thumb, then the 3rd string with the index finger, then the 2nd string with the middle finger, and finally the 1st string with the ring finger. This would be one complete front roll, and you want to repeat this pattern over and over in succession. Written out it would look like this (using an E major chord):