All of the melodies you have played so far have involved a downward pick motion, indicated by a a. With the introduction of eighth notes, the technique of down and up (s) picking is used. This is called alternate picking, and is essential for the development of speed and accuracy.
In alternate picking, use a down pick on the beat (the number count) and an up pick off the beat (the "and" count).
The following example contains the notes G and A on the 3rd string played in quarter notes. Notice that there are four quarter notes in each bar. Play the example slowly and evenly, counting out loud as you play. It is also a good idea to practice naming the notes out loud as you play.