Once you accomplish the A guitar chord, you will be able to play thousands more three chord guitar songs which are played in the key of D, using the D, A, and G chord.
Notice in the chord diagram that the second finger is positioned slightly behind the first and third fingers at the same fret? As a beginner, when you learn the A chord it's hard to try and keep your first three fingers directly beside each other because of the tight fit. By keeping your second finger a bit further back it's easier to fit your fingers.
Follow the diagram and strum the A chord, using only the five strings. Strum up and down. Practice lifting all your fingers and replacing them in a group, until you easily remember the A guitar chord.
Now play the D chord, then A, then D etc., until the changes become easy.
Next follow the same procedure using the A and G chords.
It's time to have some fun playing guitar in the Key of D. Make up some of your own music. There's music in you, I know that for a fact! You wouldn't want to learn to play guitar if there wasn't. Play, and have fun! It doesn't matter at this point if you are perfect, it does matter that you enjoy yourself.
Here's a tip. About ninety percent of the time, when a song is played in a certain key, the first chord of the song is the one that the key is in.
The first chord is usually also the chord that is used most often in the song. In the key of G it would be G, in the key of D it would be the D chord etc. So, if you jam with someone, you can usually tell which key a song is in by the first chord.
Lets move on now to the next easy beginner guitar chord, that's used a lot in blues, the E chord: