There are a couple differences between a major and a minor chord.
Sound: Minor chords are often described as sounding “sad.” and major chords as sounding “happy.” With a little practice you can begin to quickly distinguish between the sound of major and minor chord.
On the keyboard: The only difference in the playing of the major and minor chords is the position of the middle note of the chord, or the third. In a minor chord, the middle note is a half step lower (flat) than the middle note in a major chord.
C Major chord = C E G
C minor chord = C Eb G
Therefore! If you know A major, you also know A minor. Just lower the middle note a half step! If you know E minor, you also know E major. Just raise the middle note by a half step!
I hope your chord vocabulary just doubled!