Spacing. If there is one thing that will increase your game tenfold, it is spacing. Properly spacing out your attacks is the difference between them punishing your attack or them whiffing.
Secondly, stay away from certain characters. If you really want to learn the game, focus on fundamentals and play characters that heavily rely on those fundamentals to win. I'd stay away from any super small, super fast, combo characters like the Chu's, I just see too many **** players with them online just going HAM. They have no fundamentals, they just run around constantly pressing buttons, relying on being, small and fast. By and large, against bad competition they'll get away with it. But once they start facing better people they get creamed because they never learned to space properly, when to jump and what button is block.
a lot of people suggest a swordie as spacing is their bread and butter (lucina). I agree with it to an extent, although by and large she is still fast and has great hitboxes. However if you don't space your aerials and tilts correctly you'll consistently get grabbed and naired until you learn your lesson.