This is what I tell players totally new to PR:
Join the
infantry squads, and choose the rifleman class.
When shooting to kill, the amount of time you wait after aiming down sights is the single most important thing determining your accuracy. In this game, it takes a
long time for the soldier to "steady" his weapon. On the M16A4 rifle, it's about 3.5 seconds. It is very very difficult to tell exactly how "straight" your weapon is just by looking at your screen. What I do is I aim down my sights, then wait at least 4.0 seconds before firing any rounds.
If you shoot any earlier than 3.0s, the gun may as well be shooting into the sky. The heavier the weapon is, the more time it needs for the soldier to "steady" it. For a medium-sized machinegun, I think 7.0s is about right. On a purpose-built sniper rifle, probably like 12.0s.
The sound of your gun firing is like an insanely strong magnet that draws the enemy towards you. Firing in full auto makes it very easy for the enemy to tell where you are shooting from, and what kind of firearm you are shooting with (also tells them if you are an enemy or one of them). But a single shot makes it a lot harder. It takes them a while to realize you are the enemy, and even longer to realize where you are shooting from. Whenever possible, only pop off only single accurate shots to keep them guessing. Full-auto is best reserved for close quarters combat.
Ask your teammates what the best way to identify the enemy uniform is. Don't stray more than 100 meters away from your squad leader. Shoot at what your teammates shoot at.
The rest is learning in-game from teammates via voice chat. The manual is certainly worth reading, but it cannot and does not cover every aspect of actually playing the game. Using your voice-chat and asking others how to do certain things is the best way to learn.
Enjoy your stay!

I've played a lot of "realistic shooters" but for whatever reason this 2005-era mod still provides the gameplay that I find most enjoyable.