To play the A note, pick the open 5th string.
Play the B note with the second finger of your left hand behind the second fret of the 5th string.
Play the C note with the third finger of your left hand behind the third fret of the 5th string.
A short line placed beneath the staff is called a Ledger Line.
Here is an example which uses A, B and C on the 5th string along with other notes you have learned. Notice the use of minor chords in the accompaniment.
The following example is a 12 Bar Blues in the key of A minor. It uses notes on each of the first five strings and also contains ties and eighth rests. Take it slowly and count out loud as you play until you can play all the rhythms accurately.