Place the tip of your middle finger of your left hand just above the third metal bar on the fingerboard, called a fret, on the top string, which we'll refer to as the sixth string. Place your index finger above the second fret on the next or fifth string and your ring finger above the third fret on the first or bottom string. This forms the G chord.
Press the strings down firmly. Strum the guitar and listen to the chord. If you notice that any of the notes are muffled or buzzing, make sure that you are not touching any of the adjacent strings and that your fingers are above and not on the frets. Practice this chord until you can play it smoothly and clearly.
Place your ring finger above the third fret on the fifth string, your middle finger above the second fret on the fourth string and your index finger above the first fret on the second string. This forms the C chord. Practice playing this chord.
Place your index finger above the second fret on the fourth string, your middle finger above the third fret on the fifth string and your ring finger above the second fret on the first string. This forms the D chord. Practice playing this chord.
Hold a G chord and strum the guitar. Change to a C chord and strum the guitar again. Now change to a D chord and strum. Now back to G. Practice transitioning between these chords from one to another until you can move from chord to chord smoothly and quickly.
Practice playing a simple song with a G, C, D chord progression. Strum a G chord four times, then a C chord for four strums, then back to G for four strums, then a D chord for four. This is the first half of the song.
Strum G again for four strums, then C for four, then G again for only two strums, then D for two strums and back to G for four to finish. This is the second half of the song. Try varying the strumming patterns in the song to add interest to the song.