Once the shields break on the enemy, you see like electricity like animation around their body. That's when your weapon will do bonus damage to the head, and when your precision weapon (like the magnum, dmr, carbine, light rifle etc.) Will grant you the head shot kill.
So for example...in a 1v1 if you and your opponent land 4 shots to each other and you shoot your 5th shot and it lands on the body, that won't kill your opponent. Meanwhile if he shoots you in the head for his 5th shot, then that kills you.
Its slightly different for each weapon. The magnum, dmr, and unscoped light rifle are 4 shots to the body, 1 to the head. (Light rifle is stronger scoped in) you get the perfect kill medal for executing the 5 shot kill.
Br is 3 to the body, 1 to the head.
Sniper type weapons of course are 2 shot to the head.
You should check out the youtuber Aozoli if you want an in depth look at kill times, how many body shots, what grants the perfect kill medal etc. He breaks down all the weapons in a very detailed way.
Aside from that...its a team based game. So being close to a team mate or supporting from a safe alternate angle would be good until you're confident in your 1v1. Its a tough game even if you've been playing for awhile. For me, I love the game but since I don't play often now it's made me suck pretty bad at it. I was not too bad before but now I struggle to do average lol.