How can you make a harmony(in music)?

like how can you make a harmony part that goes good with the melody part

Oh singing?? If thats what your talking about just try singin the higher and lower parts of the song and find the harmony. Its nit that difficult but you have to have a good ear for it. It comes naturally to me but some people cant get it and other people it just takes practice :)

You should have knowledge of music theory and an understanding of what cords and progressions would be appropriate for the style of song.

In the most general sense you choose chords that go with the melody being harmonized. Chords on average change every measure but they can change more often as well as less. The notes in a melody imply certain chords though melodies can be harmonized more than one way.

I wish I could be more helpful but it requires skills that only come from practice and study. If you are studying guitar or piano you should ask your private teacher to give you some instruction in this area.

but what if the composition has eighth notes, the cords will change eight times every measure

Stick to a certian scale and go off of that?

Probably not. There are passing notes as well. Chords do not change along with every note in the melody. The most common frequency is every bar or measure but your ear should guide you.

At this point you should look at some sheet music with chords for a song you know. This should help you get a feel for chord changes.

with different chords at one time… it depends what instrument for the notes.

you either need two singers, or two instrument, or a singer and instrument, and the voices go together to sound the same(notes).