When using chords to harmonize melodies, certain chords are more common than others. The most commonly used chords in any key are the tonic (1), subdominant (4) and dominant (5). With these three primary triads, it is possible to harmonize any melody in a key, because between them they contain all the notes (degrees) of the scale, as shown next.
Here are the primary triads in the key of C major in voicings that are easy to play on the keyboard.