Forenote Infantry combat is as close to the standard shooter as it gets. The closest game to PlanetSide's Infantry combat is probably the Battlefield series, or Call of Duty.
There is so much to “skill” in the FPS genre, and many of the components are only able to be learned from playing. That said, there is a lot that can be learned from reading and viewing others who are experienced. My personal suggestion though, is that you start from outside of the game - the physical setup of your desk, you chair, your mouse and your keyboard.
The next step is to improve your computers environment for gaming as best as you can.
PlanetSide 2 has decent aiming mechanics. That said, the weapon sights themselves are often slighly inaccurate.
“Sights” in PlanetSide 2 can be terrible off. Bullets are fired from the dead center of the screen, while weapon sights like the 1x Reflex can be off by a drastic amount. Worse than that, is that this is not constant, and varies with a randomly animated pattern.
Aiming is the least reliable skill in first person shooters. Some days you're on fire, others you're not. There are though, steps that can be taken to help.
Crosshair placement is about where your crosshair (center of screen) lies while you are not shooting. Essentially, you force yourself to be always aiming at something.
If you are running towards and moving around a corner, keep your crosshair on the edge and “pie” it. This is an almost orbital action. If you catch an enemy, stop and engage while its only a 1v1. When approaching or defending entrances, be decisive and aim where enemies are like to come through. For example, if there is a doorway, line up a headshot on one side of it (ideally the one enemies are most likely to enter from).
With most infantry weapons you should aim for the head. This becomes more important the more accurate the gun is. Guns with higher accuracy often pay the price in terms of overall DPS, but by getting headshots this can be accounted for.
There are virtual no weapons in PlanetSide 2 that use “hitscan” (instant damage bullets). You must lead every target according to its distance from you, the direction it is moving, and you must judge this according to your weapons bullet velocity and drop.