Have you played through story mode? I'd recommend playing through it a few times if you're struggling to get the basic concepts of the game. That's what it's there for, after all.
Game developers are pretty clever people. Rather than giving you an instruction manual to read through, they design the game levels to teach you how to use basic game mechanics to perform more advanced techniques. I'd recommend playing the story levels again and pay attention to what the developers are trying to teach you via the techniques you have to use to clear each level. Mix and repeat until you can run through each level smoothly (the Octoling levels can really help ease into multiplayer). This will help ingrain some gaming "muscle memory" that simply reading online guides won't. After you develop a "Splat Sense," you'll find it much easier to react appropriately to online situations.
Just $0.02 from someone who just started online play yesterday after finishing story mode.