When it comes to getting better, time and experience is the best teacher.
Sometimes the situation will call for a change. This can be either a change in weaponry, a change in approach, or even a change in play style.
One lesson is learning to use the terrain to your advantage. Cover is key to avoiding bullets. Knowing your terrain is also key to flanking. Always go for the upper hand, never do the same thing twice in a row.
Messing with mouse sensitivity can also help. Experiment so you can figure where you flick toward the enemy best, and which helps you maintain accuracy.
Learn when to use marksman mode, and when not to, when it comes to long and close range.
Another way to get better, is to play with friends, or group with people you like. You can learn a bit about everything by playing with them.
I'll say again though, time and experience will teach you what you'll need to know.