Where do I start?

I've had this idea of composing music all my life, but I really haven't thought of it in terms of just jumping into it, but instead thought you just go to music academy to learn those things. I'm currently studying for a math degree and don't think I'll have time to learn music the usual way, so I thought about learning. Maybe from courses on the internet? Or is there a site that teaches all these things.

My focus is just composing classical music, focusing on piano. Any recommendations on where to start?