When it comes to combos, you should be training until you can't fuck it up. Same goes for characters, learn their ins and outs like everyone else will be doing.
You should be experimenting with hitboxes to see what hits where and from how far(as well as how safe it is), crossups, anti-crossups, AAs in general and giving yourself a general idea on the fundamentals of your character.
I'd recommend heavy experimentation with meter functions too. Bounce combos, extensions, all that jazz. Finding BnBs with interactables like in the background is important too.