The G guitar chord is the most commonly used chord ... by far.
"G major" chord is the full name, and it uses all six strings.
Play the G chord using your second, third, and fourth fingers as shown in the diagram.
When you press down on the strings, place your fingers just past the frets, toward the headstock. Don't press directly on the frets.
The chord diagram on the left is for right handed players, and, the diagram on the right is for left handed guitarists.
You may be tempted to play the G chord using your first three fingers because it feels less awkward, but it's slower to change between chords that way.
When you first learn guitar chords, everything will feel awkward, so, you might as well get used to the best way.
To play the G guitar chord, fret the 5th string at the second fret using your second finger.
Next fret the 6th string at the third fret using your third finger.
Finally fret the 1st string at the third fret using your fourth finger. The three middle strings are played open (not fretted).
Strum the chord downwards and slowly, to identify your trouble spots.
Listen to each string, they should each ring clear and long. If a string thuds, thumps or buzzes, either:
Resolve the problem with each string individually and keep at it until all the notes ring clearly for four seconds or more while you hold the strings down.
After you get good sound from the G chord, take your fingers off the strings, then put them back into position and play G again.
Repeat placing and removing your fingers until you remember the finger positions easily.
Once you easily remember the G chords finger positions repeat the exercise for five minutes and no longer.
It takes time to get used to all this, it's the same way for everyone when they learn to play guitar. Don't give up, you will succeed by keeping at it.
The act of pushing down on a string to produce a note is called fretting. From this point onward I will use the words fret, fretted, and fretting to describe the action of pushing down on the string. I will also be using the string names instead of the string numbers.
For the rest of the chord change lessons you should have a metronome. You can download your choice of several, good quality free metronomes from my free guitar tools page here:
Free Guitar Software Downloads
It's time to move on to the next most frequently used guitar chord: