How can I learn to play Harmonium as a beginner?

The first thing you require is Harmonium. If you don’t have it yet and still want to learn then download Harmonium app from google store  . But I would suggest that if you seriously want to learn, then go for the physical harmonium.

Now, just have a look at the keys of harmonium:

There is a same pattern of 12 Black & White keys which repeats. The first set of 12 keys is called Mandar Saptak (lower pitch), set of middle 12 keys is called Madh Saptak (medium pitch) and set of last 12 keys is called Taar Saptak (higher pitch).

Options available for you:

1. Online Recorded Videos in YouTube
2. Websites with step-by-step instruction
3. 1 on 1 Training with Harmonium Teachers
4. Offline Training

Out of these what's NOT A GOOD method to learn?: YouTube videos and websites that offer written set of instructions.

Reason is simple. You need a master who can create a custom training methodology catering just to you. You can't have a one-size-fits-all approach while learning Harmonium.

With all due respect to websites and individuals who have taken tremendous time to make harmonium basics public, you cannot learn to play harmonium unless someone else listens to you and makes corrective actions, ideally a master.

It's ideal to go through YouTube videos and other websites at first, then draw inspiration from the amazing harmonium players out there and find an amazing harmonium master for yourself.