Pick up a second weapon immediately regardless of what it is
Grenade either to start a fight or to end a fight.
Try to get a feel for the power weapon and major equipment [overshield/active camo] spawn locations and timers and try to position yourself to contest one when it’s up. Even if you don’t particularly want the weapon itself, at least denying the enemy team those things is better than them having them.
Empower yourself as the player as much as you can on respawn. This means try to pick up some equipment and a different weapon before engaging. You want the edge in a fight wherever possible, and you’re more proactive in the match in this way.
Try not to get hung up on the score when you’re losing or going negative. The game is all about the trend lines throughout the minute to minute of the match. One successful push for the power items and time to set up for enemy respawn can drastically shift the fate of the match.
Play the campaign on legendary and you will emerge a legend. No joke, I am a completely different person after going through that experience. I now know how to properly use all weapons, and end up being in 1st or 2nd place. Literally PvP feels easy compared to the pain the campaign gives you
Go and do every single weapons drill, then practice in practice area
Don’t forget to use the equipment. They are strong. 
Yeah i feel you there! Omy i understand the csrbine repeater
that was my go to on legendary!
Use the positions of your teammates to work out where enemies are.
I can now hit most commando shots and can actually use the carbine too. So from my POV, legendary is the best training method you can have in this game 
Just practice with the bots in Custom Games. That will help!
Play as a team. Doesn’t matter if its two of you or 10, just play together and focus the same target. The game revolves around team-shooting and helping out, it’s NOT a solo game. Unless you are in-humanly good at the game, then you’re just carrying.
Learn in wich situation a weapon or ability shines/how it works (techniques like using melee right after a shotgun shot upclose to kill confirm) and in wich it laks. If you dont know how to counter something, Go through your options depending on the map and change tactics according to the situation or get a understanding of wich fight to pick according to youre loudout.
Example, Bulldog and Sidekick? Dont even try to fight that BR down the map, get up to him fight him and up close or pick up a Commando. (i know thats a really simple example but I think it gets the point across)
Play the yoinking objective on the sake of the rings!
Whenever you spawn, pick one of your team mates and follow them, even if they aren’t acknowledging you, pretend like you’re moving together. Two guns is always better than one. Do not follow close enough to get both of you caught in one explosion.
And don’t forget to use the directional pad UP to Ping/Highlight enemies, places you want to go, or weapons/vehicles to your entire team to make them aware of any possible situation.
Click heads after breaking shield.
Don’t throw grenades when a bullet would kill faster
Get guud jk lower your aim sensitivity practice nade placement in general watch better players you can pickup on small things that come natural to veteran players
Throw your nades to get enemies to leave their cover, not to deal damage to them or to secure a kill running away.
When using the AR (depending on range) after the shields pop - cease fire for a second to let your accuracy reset before you begin firing again for the kill.
(I’m still unsure if this game has headshot multipliers for the AR like Halo 5 did.)