To begin, I would like to start off with some visualization. Please refer to the chord diagram below.

Credit: http://www.greenville.k12.ny.us/mschool/Kelly_P/science8/guitar/guitarchart_G.gif

Try to think of the grid area of this diagram as being the neck of your guitar, Each vertical line represents a string. Each horizontal line represents a fret. For an example, The second finger (or the middle finger) would be on the third fret of the top low E string.

At the top of this diagram, We can see the letter "G". This is the chord we will be playing.

Below this, we can see a sequence of numbers. 2, 1, 0,0,0,3

This gives us 6 numbers in total as there are six strings on a standard guitar.

These numbers refer to the fingering you should use.

For example, the first number is a 2 which sits atop the low E string.

This tells us that we should use our second finger (or the middle finger) placed at the position of the black dot on the same string.

If we count Downward, we can see that this dot lies on the low E string of the third fret. This is where that finger should be placed.

The next string over (using what we have previously outlined) Tells us that the first finger (or the index finger) should be placed on the second fret of the A string.

Moving on we can see a lot of zeroes, This simply means that these strings are played open.

Last but not least, Your third finger (or ring finger) should be placed on the third fret of the high E string.

Now with all of your fingers in place and pressing down on the aforementioned points that are illustrated by the diagram, Give your guitar a nice strum.

Congratulations! You have just played a G chord on your guitar!