A chord is three or more notes played together. The simplest is a triad: a root note, then the note two letters up (the third), then two letters more (the fifth). On white keys it's every other key: C E G, or F A C.
Count half steps to tell major from minor. A major chord has four half steps from the root to the third, then three to the fifth: C to E is four, E to G is three. A minor chord swaps them, three then four: A C E. Major chords sound bright; minor ones sound softer and sadder. Lower the third of a major chord by a half step and it turns minor: C E G becomes C E♭ G.
Most simple songs in C use three chords: C major, F major and G major. Play them with fingers 1, 3 and 5 of either hand.
Here you can play the notes of a chord together, or one at a time in any order if you're using a mouse.
Practice
C, F and G
Play each chord. Together sounds best, but one note at a time counts too.
C3DEFGABAC4SDDEFFGGHAJBKC5WETYU
Computer keys: ASD… for the white keys, WET… for the black keys, ZX move an octave.
More info
Tips
Drop all three fingers at once, from just above the keys, so the notes start together.
Playing the exercises
Use a MIDI keyboard (Chrome, Edge, Firefox or Opera), the computer keys (ASD… for white keys, WET… for black keys, ZX to move an octave), or tap the keys on the screen. A key lights up after a couple of wrong tries.
The answers
What notes make a C major chord? C E G. Root C, third E (four half steps up) and fifth G (three more).
How is a minor chord built? Three half steps, then four. Minor is 3 then 4 (A C E). Major is 4 then 3 (C E G).
What notes make an A minor chord? A C E. A to C is three half steps and C to E is four, so A C E is minor.
Which three chords fit most simple songs in C? C, F and G. C, F and G major are built on the first, fourth and fifth notes of the C scale.
What does lowering the third of D major (D F♯ A) give? D minor (D F A). Lowering the third by a half step turns a major chord minor: D F A.