I understand harmony is described as two or mote notes played togoethether. But what makes a harmony, like there's got to be certain notes that fit with eachother nicely, like for instance how would i harmonise the major scale? Im simple terms....

Thanks alot guys,
Harmonizing in thirds and sixes is the simplest way of looking at it. Once you get into the other intervals there are tons of rules that you have to follow...or ignore. Your choice.
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There's different ways to harmonize. The ones generally used are 3rds, 4ths, 5ths, and 6ths(3rds and 6ths being the most common). Based off what BGC gave you, you should be able to piece together how to do 4ths, 5ths, and 6ths. Play the harmonies simultaneously(duh) to hear how each one sounds.

Also, other than just intervals, a lot of harmonies are based off of chords. For instance, you are presented with a melody(in C major) goes C D E D and on that last D there's a G chord. To harmonize it in thirds would be to play an F over that last D. However, this would change the color of the chord(making it a G7) as F is not a chord tone of G, so lots of people would opt to play a G instead to retain the chord's character.