Download the tampura android application. Start the sound keeping the pitch as C if you are a male singer and 'G' if you are a female singer. For young boys, below their voice breaking age they should choose 'G'. If they are older then based on their comfort (a bit of trial and error may be needed for this correct choice) they can choose between C, C# , or D, or E.
Starting singing a note basic note ' Saaaaaaaa' in unision with the sound of the application.
Listen closely and you will get another sound in the tampura droid, which is the PA note.
Now try 'Paaaaaaaaa' in unision with the second note.
Now try listening to the higher octave form of the Saaa, that is the first note.
Sing that higher note 'Saaaaaaaaaaaaaaaaa' in the higher octave.
Keep singing this ' Saaaa(low)- Paaaaa- Saaaaa(high) - Paaaaaa - Saaaaaaa (low) ' a few times.
Congratulations, you have done your first self lesson in Carnatic music basics.