The term "music" means many thing to many people as its not a fixed non-changing thing. There are many scales world wide where scale means a sequence of notes. Western music, the music most people in about half of the world accept as normal is based on the concept of 12 specific notes to each octave.
In a simplified history of music a few centuries back the earliest music instrument found so far was in a cave in Europe was most of a flute made from a bone. The holes in it were spaced to give a pentatonic or 6 note scale. A large majority of the music these days is actually a pentatonic scale. Later the scale was expanded to 8 notes. Now what notes by today's definition you get just depends on where in the pitch range you start that scale. This is where the concept of modes originated as some way of getting everybody playing and singing the same notes as far as pitch goes. Consider ANY form of sound recording didnt exist before about 100 years back. So you had only 2 ways to learn a tune, by ear or off paper. So sheet music was invented as a way to define the sound of a tune. Today you will still use one or both ways to learn a tune but the difference is you DO NOT necessarily need to go somewhere to hear music. Just pop in a CD or turn on the mp3 player. Over the years each change to music has incorporated the older forms. Today an octave is a series of 12 notes defined as a specific frequencies.
You will almost never hear any music that actually uses all 12 notes in a tune but that doesn't mean it dont exist. Most music today uses only 8 notes out of the 12 but there are a large number of tunes using only 6 notes. Amazing grace is most likely the best known 6 note tune. Count up the note names sometime.
Today modes still exist and are used. What we call a major and minor key are actually 2 of the modes that are a sequence of 8 notes. The only difference between a major or minor key is HOW you work out the scale from the 12 notes. A typical diatonic fretted instrument selects the notes automatically for the major key for whatever the string is tuned to. Its exactly the same pattern you see in he white keys on a keyboard. The black keys are the "missing" notes in the 12 note sequence. A minor key uses almost the same pattern with a small change. Notice there is 2 wide spaces on a dulcimer after the nut and winds up with a narrow space (assuming you have the 6+ fret) at the octave shift. That is a major key. A minor key starts with just 1 wide space and winds up wit a wide space before the octave shift. In essence what you do is move the first wide space from the start to the end of the octave.
With a dulcimer tuned open DAd its in D major. Now capo it at the first fret and you have E minor and the scale goes one note higher than the next D, the E. Notice what happens, that wide space removed from the start winds up just AFTER the 7th fret. By the way the same thing works on a fully chromatic MD or guitar too but you have far more possibility for keys using a capo.
That being said how you work out the notes for a chord is exactly the same. Start on a note and just skip every other note until you have all 3 notes you need. Its the same thing as defining a chord by steps but its far more graphic if the concept of interviels isn't sinking in.. There is loads of stuff on the net that goes into great detail on chords such as chord progressions and so on if you're interested.
rendesvous1840 is right in how to use chords to set a feeling to a tune. If you want a typical feel good happy tune then just stick to the first 4th and 5th chords. If you want something a bit on the sad side then use some of the other chords. You will find a lot of this in many "church" songs. If you can read music to a minimal degree check out most any hymnal. The metal heads often use 2 notes side by side as a form of chord to make you feel edgy. Music is loaded with all sorts of things like that so take some time break free from being confined by music theory. Music theory should NEVER be seen as some set of absolute rules, but more like how modern music works or as a starting point in making music.