I figure that whenever I make a long response to a question about how to improve an aspect of your game on another board, might as well re-post it here. And yes, the tl;dr is, infact, practice practice practice.
"Honestly, take this from someone who knows. There's only one way to get good with plasma (and yes, it is possible): using it. Using it a lot. All the theory in the world won't help your plasma aim if you don't actually spend lots of time working on applying that theory. For the record, you should use it as if it were a cross between the RL and LG.
I used to play CA with a guy called Crucci, before he switched to FFA (not sure if he still plays). He would sometimes play entire matches with only the plasma, which pissed his teammates off, of course. Mind you, this guy is easily as good as, if not better than me. Thing is, his PG alarmingly quickly got better and better, and he ended up turning into a plasma demon. Here's an example of the type of stunt he would end up pulling off with alarming regularity: http://youtu.be/Df0NVOCNiKE?t=6m29s
This applies to most weapons you want to get good at: use them. A lot. Even in situations where you really shouldn't. That's how I finally fixed my crappy rocket aim. By unbinding the LG.
By the way, since you're new I really wouldn't bother trying to learn Plasma. Start by learning the Railgun or Rocket Launcher. I'd suggest learning the Railgun since it's quite easy to practice. Do 5-15 minutes of this a day: https://www.youtube.com/watch?v=gZ1plnx0Hco and then play vs regular players on your gamemode of choice (that isn't instagib). I started a CA practice server for that video, but I got the tip from a great player that starting an FFA practice server is probably the better choice, since you can add more bots without it turning into chaos and the situations will be a bit more realistic, where you'll be shooting enemies shooting at other people. Also, you'll learn where items spawn.
And not only is it easy to practice rail, it carries over to all other weapons, including the Rocket Launcher, since it teaches you the very basic skill of "moving your crosshair onto your target", which is a skill required for aiming with any weapon, even if you're using projectile weapons like Rocket Launcher, since you still need to be able to put your crosshair where you want your rocket to fly quickly and accurately, even if it's not always on your opponent. Also, for the Rocket Launcher, the chief mistake I see beginners make is over-predicting or over-leading your target. It's more productive to aim right for their feet than flick your mouse far to the side to try to predict where they're moving. The latter will generally make your rocket miss by a large margin whereas the former will at least give you splash and quite often a direct hit, since people like to dodge side to side and often end up moving right back into the rocket. For RL mastery, the thing you need to master is patience. Going against your instinct and not firing right as the reload time is up, but instead waiting to see where your opponent goes is the best way to consistently hit direct rockets. It's difficult to restrain yourself from pressing mouse1 every 800ms, but oh so worth it.
Also, it might help using proper mousing technique, especially if you don't have a gaming grade mouse and pad and/or are using high mouse sensitivity: https://www.youtube.com/watch?v=X0Ioc5RNCjI "
By the way, crucci was also amazing with the Shotgun, and guess what? He would play entire matches using only the Shotgun as well.