So i’m a relatively amateur photographer and right now I’m starting to build my portfolio by an instagram account.
What that said, there is no chance in hell you're ready to shoot an NBA game.
You want to get there, start by getting good and not considering yourself to be an amatuer.
I'd tell you to shoot small basketball stuff, but from what little I know of sports photographers, that's not a way into that world. Being really good, becoming an assistant to an NBA shooter, and then when they bow out, you being in the position to get that spot, is how you get it, last I knew.