The C Major 7 Chord is found by locating the 1, 3 , 5 and 7 positions (C, E, G and B) of the C Major Scale: C D E F G A B C. You are basically playing a C Major Chord and simply lifting your first finger off the C Note of the 2nd string first fret. That string now becomes open, so you are playing a B note. The chord chart below demonstrates how to play the CMaj7 Chord.
Here is a key that will help you read the chart:
C Major 7 Guitar Chord Video
The video below will further demonstrate how to correctly play the CMaj7 Chord. Enjoy!