I used to suck too, until recently, so here are a bunch of tips that worked for me:
1) Get used to the controls, whether you use Motion or Analog (the non-Motion control option, I think that's it's name). This is critical; after all, nothing else matters if you can't control your Inkling!
2) What kind of weapons do you like to play, and what playstyle are they geared toward? For example, if you like Chargers (if you haven't unlocked Ranked yet, they get so much better in the Ranked modes, it's not even sort of funny), you should probably hang back and snipe from a distance, and try to avoid close combat, whereas if you like Rollers, you should lead the charge, inking up areas in a timely fashion and swinging at enemies to keep them at bay, but avoid midrange combat, since you'll lose. Figuring out what you do and don't enjoy using is important, since trying to play a weapon you don't enjoy will get dull quickly (that's basically my experience with the Tentatek Splattershot; it's good, and I usually win when I use it, but I don't really feel like I'm enjoying Splatoon when I use it).
3) Pick a single weapon and try to get good with it. You can switch if you want, but mostly try to use that one weapon until you feel you've mastered it.
3.5) Also, try new weapons as you unlock them. You might find something in the recent unlocks that you enjoy using a lot, and it might become a new addition to your arsenal. It's happened to me quite a few times, with the Splash-O-Matic, the Octobrush, and the N-Zap '89, to name the first few I can think of.
4) Speaking of new weapon unlocks, PLAY OCTO VALLEY. It's fun, it can help you get used to how Splatoon plays, and you unlock a bunch of weapons (and some gear!) by beating the bosses of Octo Valley, so you should do it.
5) SUPERJUMPING IS NOT REQUIRED. YOU DO NOT HAVE TO DO IT EVERY TIME YOU RESPAWN, YOU'LL JUST JUMP INTO AN AMBUSH LIKE HALF THE TIME. I still see this in A- Ranked lobbies (and I still do it), so I feel it's important to stress that.
5.5) Superjumping isn't useless, despite what I said above. If you know it'll be a safe superjump, or you have Stealth Jump (but that'll be slower than usual unless you have a Quick Super Jump to counteract it), or if you're jumping to a Squid Beakon (you don't get a warning that someone's jumping to a Beakon, so it's like a Stealth Jump without having to equip that ability), you should do it if you don't have anything to do where you currently are (and won't in the near future).
6) Be patient. You don't have to rush in constantly, especially if you're winning. It'll probably just get you splatted by someone who's waiting for that, and despite splats not directly counting toward the objective of any mode, getting splatted still hurts your team's chances, if only briefly, of winning, whether by holding onto a lead or making a last-minute comeback. It's better to move in with your teammates or shoot from a place where you're not likely to get splatted, and then advance a little (not too much, don't go very far beyond center unless it's a last-few-seconds push, generally) and secure the area.
7) Try to avoid getting splatted when you can. Getting splatted causes an inksplosion of the other team's ink to be spread from where you were splatted, which lowers the amount of turf your team has covered on the map and gives the enemy team a bit more area to swim in, and while you're waiting to respawn the rest of your team may get ambushed by the guy you were splatted by, since they have the numbers advantage over your team (at least, sometimes) and can gain a brief period of total control of the map if your whole team gets splatted.
7.5) Having said that, if you can splat at least two members of the enemy team before getting splatted, it might be worth it to take one for the team (and if you can somehow get the whole enemy team at once, go for it).
8) If your whole team got splatted except for you, try to retreat. You do nothing for your team by getting splatted and handing the map over to the enemy, so try to avoid getting splatted until they come back.
8.5) Though if you can't escape, try to splat your attackers before being splatted. At least you might take someone down with you, and they can't ink as much turf while your team gets back into the fight if there's one fewer of them.
9) You will get put with teammates who suck worse than you do, and sometimes you'll just end up against someone who wrecks you and your whole team for free. It happens, don't let it get you down.
9.5) On the same note, disconnects happen. Try not to get too annoyed when they happen to you or your teammate and you lose because of it (though if you win despite losing a teammate, that's awesome, and doable, so don't give up!). On the other hand, if it happens to the enemy team, wreck them extra hard.
Sorry if that's a lot of stuff, I just wanted to put everything I thought of. And sorry if some of that is fairly obvious stuff.
Hope it helps!