To pick the strings, use a downward motion of the pick (called a downstroke and indicated by the symbol a), and use only the tip of the pick to strike the string, as shown in the diagram below.
Shown below is a picking exercise designed to help you gain control of picking notes on all of the strings. Take it slowly at first and be sure to keep all the notes even in both tempo and volume. To develop a good sense of timing right from the start, it is important to use a metronome (or drum machine) every time you practice. It also helps to tap your foot on each beat and count out loud as you play.
Use the tip of the pick to play the string.
This exercise involves all the open (unfretted) strings played as quarter notes. Count 1 2 3 4 1 2 3 4 etc. as you play, using all downstrokes. The two dots before the double bar at the end are a repeat sign. This indicates that you play the piece again from the beginning.