I need some help. how do create a hoarmony?
For Example- "Twinkle twinkle Little Star" ?

Harmony can be difficult, especially for beginners, but if you remember just one basic rule, it's quite easy...

Take your base notes - the ones that fall in the beginning, the middle and the end and just harmonise those using the base note plus two more - use 1st (base note) 2nd (third note) and 3rd (5th note)

So if you do twin-kle twinkle lit-tle star in the key of C, you get c,e,g as the first note (the harmonisation falls on the 'twin' of twinkle) and the 'lit' of little would be f,a,c, then back to c,e and g for 'star'.

Basically you're making the chord sequences using base notes as a starting point.

have a look at the piano theory under triads, that explains the basic principles of harmony!