The guitar has 6 strings and in standard tuning, we tune each string to be a perfect fourth pitch interval apart as we descend strings (save for one pair). Since the 5th string has already been tuned to A, all we need to do is tune the remaining 5 strings around that. The top string (6th) will be tuned to E, 5th string to A, 4th string to D, 3rd string to G, 2nd string to B and lastly the 1st string will be E.