When playing an E7 we put our index finger on the first fret of the G string. The middle finger is then placed on the second fret of the C string, and the ring finger on the second fret of the A string. Keep the top finger joints as straight up as possible so they do not touch the other strings. Make sure the middle E string is not muted as that is essential for the chord! As long as you keep your fingers perpendicular the E7 is nothing too difficult.