The G major chord consists of a figure making use of two of your fingers. The easiest and most common way of marking this chord is by making use of your index finger and the middle finger.
The finger positioning is as follows:
Take a look at the video below to get a closer look at the G Major Chord:
This is demonstrated in the video below:
With the G chord, it is good to keep in mind that your pinky will ultimately become a great contributor to your music. This will be possible my making use of the pinky to mark notes in the G scale in order to enhance your G chord. Try it out by marking the G chord as is demonstrated above and then use the pinky to mark the 3rd string on the 3rd fret. The effect should be to strum along with the G chord and occasionally add the pinky to give it a different twist.