Harikrishnan U

Harmony means existence of two or more notes simulataneously. When it comes to singing harmony, you should develop the skill to sing a note/melody while listening to another note/melody. A good starting will be trying to harmonize basic scales like Major/Minor scale. You can try playing the Major scale on an instrument or audio track and try singing harmony with it. Once you gain confidence, start harmonizing simple melodies/ rhymes and then later on songs. Theoretic knowledge about Harmony construction will be an added advantage while approaching the concept.

When it comes to any kind of art, the interest and dedication towards it makes the biggest difference than talent. A naturally talented person might get a comparitively easier start, but that doesn't guarantee that she/he will become a maestro. On other hand a well dedicated person who passionately work towads learning & perfecting the art will surely go a long way ! So if you are planning to start learning Carnatic music, I would recommend you to start learning the art and not to worry about talent. 

Notation is the language in which Western music is written. Although it is an important aspect, it is not true that you can only start with that. Generally it comes to the teacher's descretion that when to start teaching how to read the sheet music. A core western music teacher might introduce it in the initial days, but some teachers introduce it according to the comfort of the student.