You will never get anywhere without practice, and even if you put in a good thirty minutes a day, you’ll make a lot of progress. Almost as importantly, you need to practice effectively. I’d recommend that you learn how to play scales before you do anything else. Yes, they are boring, tedious, and I hated them when I was a kid, but they greatly improve your technical skill. Not only will your fingers learn how to move efficiently, utilizing common patterns found in all pieces, you’ll also learn how to recognize keys and differentiate between major, minor, and so on. They’re relatively simple and they don’t have many sharps or flats you need to worry about. You can work your way up from there, looking at more difficult majors and then turning to minors. You should be able to find the notes and the fingerings for them on the internet – but be sure you find the right ones! The fingerings are vital to your growth – if you use random fingers, trust me, it’ll be difficult later on (I learned that the hard way). Your hands need to get used to moving efficiently, and the appropriate fingerings were designed with that purpose in mind. I’d recommend you start with your right hand (as this is the hand you’ll be playing melodies with), and then play the scale with your left hand (keep in mind that the fingerings are different for either hand). Then, when you’re feeling confident, you can play both of them at the same time. Since they’ll follow the same notes and have a repetitive fingering, it shouldn’t be too challenging to put together. Start with C major first, though!
The harmonium requires air to be pumped through it with the bellows, using the left hand. The piano does not. Consequently, the left hand is free to play chords. On the harmonium, you can either play the melody or the chords. This somewhat limits the number of things you can do with it. The piano has a much larger scale, which opens up many possibilities. There biggest concert pianos have 88 keys. The harmonium is much smaller. However, the two instruments are used for different things, so this is a moot point. The keys on the harmonium need very little force to play. Also, varying the force produces little variation in the way of dynamics. Playing the piano requires extremely clean technique and fine control over the amount of force being applied. The weight of the keys matters quite a lot. While playing any given piece on the piano, it is far easier to play the correct notes, and much more difficult to play them the way they are supposed to be played. What does introduce dynamics into the playing of the harmonium is the ventilation technique ie. the technique used to control the bellows which push air into the instrument. At the “turnaround points”, when the bellows are either fully compressed or fully expanded, the notes produced will be the weakest, since the least volume of air is being pumped in and out the instrument. Even mid-compression or expansion, varying the force applied is a method of varying the volume and “punch” of the notes. 90% (I am being generous) of players have terrible technique in this regard (including some big names), resulting in the wrong notes sounding weak or emphatic. They are content to just cycle through it at a constant rate. This is even more important than right hand technique, and skilled players can use it to create beautiful trills and subtle variations, but this is lost on most people. No one seems to know, let alone care about it.