Hey man! Welcome to the scene! You're in for a heck of a ride!
The first thing I would recommend, and I say this because you mention coming from a CoD background, is you have to forget a LOT of what you know about shooters. This game is drastically different from other shooters and you have to treat it as such. I came into Splatoon from a background in TF2. I did well enough until the B ranks using what I knew about shooters based off TF2, but to keep going, I had to ditch a lot of what I knew from TF2.
One difference: You have to move CONSTANTLY. Watch some good, high-tier players. They almost never sit still. I recommend watching videos from ThatSRB2Dude (I've noticed Dude only likes to sit still to set up for stealth kills), Nintendome (a lot of good meta stuff here), SquidKids (also good meta stuff) SquidsNextDoor, Name Subject to Change (one of the best, if not THE best, competitive teams out there right now), and, not to shamelessly plug or anything, but the other captain of my squad, Aristeia, is a fantastic player (far better than me). You can see some of his awesome gameplay on our channel Watch The Whiprays.
Another big change is that due to the fast, aggressive type of game it is, it's VERY hard to "carry" a team in the higher ranks in Splatoon, especially once you weasel your way into the A and S ranks. Almost every single time you're going to have to have a good, coordinated team backing you up. That being said, you can't get in the mindset of blaming your team every time you lose. A LOT of people do that and it's unhealthy and unhelpful. Everyone has a different play style. Even if 2 people are playing the same weapon, they may play it very differently. So when you play ranked and lose, remember that it's likely due to your teams' play styles not meshing well. Which is a big part of why once you get into the higher ranks, especially S, a lot of people join or create squads. If you have a dedicated team you play with all the time, it gets easier to coordinate and do better.
A note on play styles: Like I said, 2 players playing the same weapon their play style may be drastically different. That's another big deviation from the typical shooter motif. In TF2, if you played Scout, you were pretty much saying "I'm going to run around with a shotgun and a baseball bat and play fast, aggressive offense". If you were playing Sniper, you were saying "I'm going to pick out one or two secluded places to shoot from and only engage 1v1 in really tight situations, I'm here for support/defense". In Splatoon, these concepts of "roles" go out the window. A Gal player can choose to play aggressively OR defensively, depending on their gear and play style. A sniper may be moving constantly and have NO fear of getting aggressive and engaging 1v1. 9 times out of 10, what you expect to see from a particular weapon is not EXACTLY what you'll get. There are certainly specific things within the meta of the game that are far more common than others (things like Run Speed Up on Splatlings, Damage Up on Sloshers, many Shooters, and even some Brushes, Quick Super Jump and Stealth Jump on Tower Control maps, Dynamo Rollers running Defense Ups, are all things you will see a LOT) But never go into a match think you know EXACTLY how it's going to go down because you don't. And you never will. And that's the beauty of Splatoon. It's not predictable or monotonous.
And of course that means that picking up one weapon does not glue you to one play style. Mess around! Have fun! Try playing hyper-aggressively with a slosher. Or play more defensively with a splattershot. Pick up weapons that are weird or intimidating to you. If you liked semi-automatic weapons in CoD, you might really enjoy the Nozzlenose series. It's tricky to pick up, but terrifying when played well.
Really, the best advice I can off you is to watch high-tier gameplay and don't get stuck in your old FPS mindset. You're in a whole different world now, honestly. Take your time. There's no rush to get to S+ or anything. And the Pro-Comp scene for this game is still pretty small. If you're interested in going pro, you've got time to unlearn what CoD taught you and learn everything that Splatoon has to offer.
Good luck and keep on splatting!