What type of material a beginner should sing?

In terms of exercises you may start with humming, lip trills, tongue trills, sirens. For scales try 5 note scale, major scale. And for songs it depends upon you. Cause even if the song is difficult you can modify it. Don't forget to download a metronome app and jam with a rhythm.

Robin could you explain what you meant by material?

swaras

a beginner should start up with only sargams..i.e sa re ga ma pa dha ni sa

This is a research work. It depends on students ability.

Sargams, alankaars, alankaar palte (different permutations and combinations of notes), long stay on base notes, one breathe stay at each notes, etc are some of the exercises.

And my answer was based on Hindustani Classical Music.