The A7 Chord is found by first locating the 1, 3 and 5 (A, C# and E) of the A Major Scale: A B C# D E F# G# A, and then adding a flat 7 note. To locate the flat 7 note, we first find the 7th note of the A Major scale (G#) and then go back a 1/2 step to the G.
So the notes of the A7 Chord are: A, C#, E and G
Here is a key that will help you read the chart:
A7 Guitar Chord Video
The video below will further demonstrate how to correctly play the A7 Chord. Enjoy!