A scale in music is comprised of 8 notes. 
C D E F G A B C  This is the scales of C Major.
This is called an Octave. 

 

 

 

 

 

 

 

 

 

 

 

A Major chord from any scale is made up of three notes. The first, the third and the fifth notes from the scale. 
The first note is the ROOT note (in this case - C)
The second note in the chord will be the third note of the scale - E

The third note in the chord will be - G