EcceLuxCrucis

Hello, I'm gonna write Mt first piece starting today. I have written non classical music before (hymns, and CCM) but I want to write some classical music since I love tchaikovsky, cui, Shostakovich, etc. I don't know where to start. What do you recommend? Should I write an outline? That seemed natural. I'm gonna write it for solo piano since that is what I'm most comfortable/familiar with. I might add a second instrument though