The major scale consists of 8 notes (acually 7, with the first note appearing in 2 octaves,). The notes are numbered above. There are other scales like pentatonic, blues, Hermatian etc. Anyway, the 1, 3 and 5 notes make the major chord. So the G major chord consists of G,B and D notes. The minor chord is made of 1, 2.5 and 5 notes (the 2.5 can also be written as 2b, where b stands for flat). The 2.5 note is the note which is between 2 and 3 notes. Notice that this note is acutally not on the scale. Anyway, 2.5 note for G is, the A# note. So, G minor consists of G, A# and D. Similarly suspended 4 chords are made of 1,4 and 5 notes. If you want to play a chord on the harmonium (I don't think people play chords on the harmonium, but if you want..) just play the 3 according notes at the same time. If you want to find which notes make c major chord, just write the 12 note sequence starting with C. Like below:
G G# A A# B C C# D D# E F F# then G G#..... (repeats) | | | | | | | | | | | | | | | | 1 2 3 4 5 6 7 8
Notice that the numbering pattern (1 to 8) is exactly same, but now the numbers correspond to different notes.
C C# D D# E F F# G G# A A# B then C C#..... (repeats) | | | | | | | | | | | | | | | | 1 2 3 4 5 6 7 8
Similarly, you can find D major or F# minor or whatever you want. Start with the note whose chord you want to find, write all the 12 notes in serial. The numbering of 1 to 8 follows the same pattern no matter which note you start from.
C major = 1,3,5 = C, E and G C minor = 1,2.5,5 = C, D# and G