How does one get better? Like when you play what areas with what guns do you shoot at? How many bullets does it take to take dpwn a shield? Any advice is helpful thanks
All of the guns you can test in warzone on yourself to get an idea of how they work. Whenever you spawn at home base, and if you have at least one other base captured by your team, there is a shield that reflects all of the shots placed on the shield. You can shoot the shield and make it so that it reflects towards yourself and take a look at how much damage it does to your shield.
In general for the starting weapons:
Assault Rifle takes 15 shots to kill, 12 if the last 3 are head shots.
Submachine gun takes 15 shots to kill, 13 if the last three shots hit the head.
Magnum takes 5 shots with the 5th hitting the head, otherwise if you hit the body I believe it takes 8.
Battle rifle takes 4 bursts if the last burst hits the head. I think it takes around 6-7 bursts if you hit the body.
Designated Marksmen Rifle takes 5 shots to the head or 7 if you hit the body ( I know that it is slightly stronger than the magnum)
Note that the headshot damage multiplier only applies once the shields are down, in other words hitting a full shielded spartan in the head 4 times and in the chest 4 times will lead to the same amount of damage with the shields being depleted on the 4th.
Playing the campaign can give a feel for how the aiming system works. In general the automatics tend to be used at a closer range than the precision weapons. Precision weapons on the other hand become much more effective if there is some distance between you and your target.
Hope this helps, and feel free to test the damage for yourself to verify that my information is correct.