There are six strings in a standard guitar. Before you start playing the G major chord through the most standard way, you need to put your guitar in standard tuning. This means that the six of your strings should make the sounds of E A D G B E in order. The thickest and the thinnest strings should make the sound of an E note. The second string from top should make the sound of an A note. The third should make the sound of a D note, fourth should make the sound of a G note and the second last string should right a B.
Once your guitar is put to standard tuning, hold it the way you do and put your middle finger on the third fret of the E string. This means that you will be playing the G note on the top most string of your guitar.
With the third fret of the first string locked down, hold down the second fret of the A string. This means that you will be playing the B note of the second string from top.
The third and fourth strings should be left open. This means that once you are going to strum later in order to play the G major chord, these two strings will be left free and they will ring D and G notes as you are not holding down any frets.
Hold the third fret of the B string, which is second last from the top. This means that you will be playing the D note of the B string while playing the G major chord.
The last string, which is also an E, should ring the G note. For this you will have to hold the third fret of the last string.
Make sure that you hold down the frets tightly and properly or you will hear a buzzing sound while strumming.
The overall notations of the G major chord are G B D G B G.
Start strumming away as you are now holding the G major chord.