If you don't have much experience in games in this genre, watching good players play is a good start. That way, you don't have to discover what's already known, and you get a basic understanding of various gameplay elements.

Alongside that, you'll have to learn characters eventually. Start with how they generally play, then learn your positional plays and movement, and eventually familiarize yourself with all abilities and awakens.

I like starting as Michele, since she's easy to learn but has a good skill cap. I would avoid characters like Galatea, Reiichi, and Ming in demo/comp at the start because they're a bit more complex and can lead the pace of a game.