To play the note G, pick the open 3rd string (no fingers placed behind the frets).
An open string note is indicated by a white letter on a black dot placed above the nut.
To play the note A, place the second finger of your left hand just behind the second fret of the 3rd string.
A fretted note is indicated by a white letter on a black dot placed just before the fret wire.
This is a quarter note.
It lasts for one beat.
There are four quarter notes in one bar of * time.
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 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. Notice the numbers on the TAB indicating the open 3rd string (G) and 2nd fret of the 3rd string (A).
Here is another example using the notes G and A. Notice the chord symbols written above the music. These indicate the a second guitar would play to accompany the melody. Try playing the melody and then swapping to the chord progression. Being able to swap between melody and chord playing is important for a guitarist. In a group situation this translates into Lead Guitar and Rhythm guitar, so it is a good idea to work at being good at both right from the start.
Remember that you can choose to read the traditional music notation (top line of music), or the Tablature (second line).