Chord are made of notes. So firts we need to know what a note is. A note is a single frequncy. When you hit a single string, it produces one note. Hit multiple strings and it will produce multiple notes. Now here is the interesting part, I guitar may have 18 - 24 frets. So you can hit one string at many different frets. Each will produce a different frequency. The higher fret you play, the higher the frequecny. So all these frequencies should be different notes, right? Well....yes and no. As they are of different frequncy, they are different notes. But if two frequncies are doulbe (2 times) or 4 times of each other, they are still the same note, but at a higher octave (octave means double). So if you play the e-string at open stings, you will get e note. At 12th fret of e-string, you will also get e note. But it will be at a octave higher. It's a little confusing, but it gets easier.
So there are 12 basic notes (they have multiple octaves over the 6 strings). At open string, the notes of the 6 strings are E,A,D,G,B,E (for standard tuning). The last E is 2 octaves higher than the first.
Among the 12 notes, we can play certain notes (say 5 or 6 or 7 notes) in a sequence that sounds nice and also has a distinct mode. These set of notes are called a scale. Songs are based on scales. There are major scales, pentatonic scales, blues scales etc.
Now let's stat with chord formations:
Here are the 12 notes: Most Guitar players start with G or A. Keyboard players start with C. I am starting with G.
G G# A A# B C C# D D# E F F# then G G#..... (repeats)
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1 2 3 4 5 6 7 8
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:
C C# D D# E F F# G G# A A# B then C C#..... (repeats)
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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 major = 1,3,5 = C, E and G
C minor = 1,2.5,5 = C, D# and G
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.
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