There are two common ways to play a G major, a three finger method (Frets: 320003) and a four finger method (Frets: 320033), both with a slight difference in sonority. In either way, the notes are a combination of G, B and D. From this point forward, the fingerings will be shown in parentheses for the sake of simplicity.
Fingering 1: (32ooo4)
Put your third finger on the sixth string, second finger on the fifth string, and fourth finger on the first string. This is a favorite among beginners, and it allows for easy change to the open C major chord. Alternatively you can finger it (21ooo3), which may be easier for players with small hands or guitars with small necks and is recommended when changing to or from a open D7 chord.
Fingering 2: (21oo34)
This uses all four fingers and makes for an easy G to D major chord change. This has a more «stable» sound than the first fingering because the note played on the open B string is a D therefore avoiding the doubling of the third. Don’t worry if that explanation isn’t clear; just remember the difference between the two chords (one has a doubled third). The theory of chords and how they are constructed from the intervals of a scale is a subject that requires some off-the-guitar learning but with applied study can be easily understood.
Use your first finger on the third string, third finger on the second string, and your second finger on the first string. Be careful not to play the fifth and sixth strings, since they are not required for this chord. At first this may feel awkward, but it will be comfortable to play. Watch that you keep your thumb low when you play this chord. You can also finger this (xxo243), which will help you to later use this as a barred, movable chord shape.