Sometimes you will need to use the right hand ring finger (a) to pick the first string. This situation will arise when the right hand is playing mainly in the second position but an occasional note must be picked on the first string.
Instead of changing the position of the right hand the a finger can be used. You may find the a finger difficult to control at first. Keep practicing until you are comfortable striking the first string with the a finger, getting a well balanced sound between all strings.
The following example is a perfect exercise for developing the use of the a finger. 12 Bar Rag is a common 12 bar Blues progression. The right hand uses a strict fingerpicking pattern (second position) with the a finger occasionally playing the 1st string.