The air that floated from the sky created the sound "S", which is the origin of Nadam. Along with this sound the akshara "A" considered to be the earliest was added to create the sound "SA".
With SA as the base, the other six swaras RI-GA-MA-PA-DA-NI were created.
Purandara Dasa, sometimes called the father of Carnatic Music, not only composed thousands of Krithis, but also created a set of repetitious practice exercises called "Swaravali", "Alankaras", "Geethams" etc which over time has become the standard lessons for the beginners in Carnatic Music. These exercises are in existence for the last many centuries and are the proven tools for any beginner.