Answer:
That is an incredibly complex question. Let me outline a few things you need to know.
Firstly, you must understand scales. For example, let's take the C Major scale because it's easy to work with. If you don't know, the C Major scale starts on C and has no sharps or flats, so it looks like this:
... C - D - E - F - G - A - B - C ...
Okay. Let's start with understanding harmony by looking at what are called triads.
The C Major scale made up of 7 distinct notes (C through B) and 7 distinct triads. What are triads you ask? They are simply three note cords that are made by taking any note in a scale, skipping over the adjacent pitch to add the second note, and then skipping over the adjacent pitch again to add the third note of the triad. Here's what I mean:
If we start on C, we will then skip over D to add E and then skip over F to add G. So a C Major Triad looks like this:
... C - d - E - f - G - a - b - c ...
So the C Major Triad consists of the pitches C, E, and G.
Okay, so far so good. What if we start on G instead? Well, a triad in C Major that starts on G is going to give us the pitches G, B, D. Which for our information could be called a G Major Triad. What if we start on D? Then we get D, F, A. How about A? The we get A, C, E.
I think you get the point. There is so much more for you to learn though! This doesn't even scratch the surface. We haven't talked about inversions, chord progressions, cadences, voice leading or and of the really cool stuff there is to learn. If you are at all interested in learning harmony, talk to a teacher of music. They will surely be able to give you
One last thing; as I have said before, a major scale consists of 7 triads. 3 of the triads are major, 3 of the triads are minor, and one of the triads is diminished. The triads that are built on the 1st, 4th, and 5th notes of a major scale are major triads. The triads that are built on the 2nd, 3rd, and 6th are minor triads. The triad that is built on the 7th triad is a diminished triad.
Try playing around with these different chords and have fun!