Now that you can play the melody with your Right Hand, we soon continue with your left hand!
A chord is two or more notes (most of the time at least three notes) that are played at the same time. ________________________________________________
Diatonic Chords
Lets start from the basic of how to form chords from a Major Scale.
Scale or Key has 7 different notes.

If we take the C major scale:
C – D – E – F – G – A – B – C
And we build a triad off of each note of the scale using only notes found within the scale, we get this: 
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Primary Chords
Among all Diatonic Chords, specially take note on Chord I, Chord IV and Chord IV , we called them Primary Chords.
Basically, Primary Chords fit in every phrase of the song! But When it comes to creating more harmonies interest, Diatonic Chords it fit better!