(Image source: https://www.vg247.com/2016/11/08/call-of-duty-modern-warfare-remastered-and-the-problem-with-remaking-old-games/ )
So when I'm not doing something with music (whether it be writing, practicing, or performing), in school, or at work, I usually tend to spend my free time playing video games. Video games have always been one of my favorite hobbies ever since I first had a Gameboy Advance and Pokemon Leaf Green (I still love the original gameboy Pokemon games). Eventually I was able to afford a PS3 and my exposure to sunlight decreased dramatically. The main game I find myself playing nowadays is Call of Duty online (specifically Ghosts as I believe some of the newer ones are somewhat glitchy and lack the luster that the older games in the series had, and also I can't afford a PS4 at the moment). I have so far spent a total of 13 days and 19 hours online on that one game (yes I realize I have no life haha). In this post I will primarily reference COD, but this information can be transferred to any first person shooter.
Anyway, on to the good stuff.
Centering/Aiming
The first most important tip I can give you is what is described as 'centering'. Always always ALWAYS be prepared to aim and shoot, having your gun with the hip-fire guide lines centered in the direction of most concern/danger. Imagine two lines going across your screen, one vertical up and down the screen directly in the middle and another horizontal exactly in the middle as well. This divides the screen into 4 sections making a cross. You want your enemy to be dead center of where those lines intersect. Keep this in mind when you're running around the map and all you will have to do is aim and pull the trigger without any drastic adjustments when ADS. Also, you need to be as fast as possible with every single movement of your character, in this case aiming down the sights. The longer it takes for you to aim, the longer the opposing player has to react. In Ghosts, this perk is called Quickdraw. THIS IS THE MOST IMPORTANT PERK YOU WILL NEED WHEN PLAYING CALL OF DUTY.
Reloading
Another vital strategy to improving your K/D is timing your reloads properly, as well as making them as fast as possible with the sleight of hand perk. THIS IS THE SECOND MOST IMPORTANT PERK YOU WILL NEED IN CALL OF DUTY. Make sure to start equipping this to every class you have with an Assault rifle, SMG, or Shotgun (you don't really need this perk if you decide you need an LMG except in specific situations which I may get to in another post).
Know Your Niche
(Image source: https://www.gamer.no/spill/call-of-duty-ghosts-windows/186008 )
This may seem like common sense, but you would be surprised at how often people make this simple mistake online. Someone without the skill necessary to handle themselves runs into the heat of battle with a shotgun or SMG in a wide open long range area. This is just asking for death. Each weapon has its own individual pros and cons when it comes to recoil and range, as well as ammo capacity and what I can only describe as 'feel'. SMGs are great at close range but lack the accuracy necessary to quickly kill any enemy away at a distance unless you have impressive skill (I can occasionally pull this off but more often than not I just ruin my kill streak).
If using an SMG, stay indoors or have lightweight to increase your speed going from building to building. Don't stay out in the open too long. When using a Shotgun, always stay indoors unless you are confident in you skills with a pistol or other secondary weapon.
Assault rifles are good all around for the most part, but as I previously stated every weapon has it's own personality and characteristics. Find what works best for you and your playing style.
Accuracy
There are two ways to instantly improve your accuracy...
Adding these attachments: Grip (highly recommended) and Red/Holographic Dot Sight (or VMR sight in Ghosts)
Shoot in bursts: No, I do not mean weapons that shoot in bursts of three every time you pull the trigger. What I mean is when you have an enemy far away, if you simply hold down the trigger your gun is going to go sky high because of the recoil. Release the trigger for a fraction of a second to help steady your aim every 3, 4, or 5 shots. This will become an immediately obvious solution to hitting targets beyond your gun's normal limitations.
Stealth
I highly recommend that if your strategy is running around the map constantly never staying in one spot and going where the action is as I do, always use a silencer. Nothing is worse than knowing the only reason you got killed simply because you shot and missed revealing your exact location on the mini map.
Practice
Play Free For All online. It really is as simple as that. Constantly having a gun fight and often losing over and over will force you to improve with enough time invested into the game. Play nothing but Free For All until you are satisfied with the results.
Fighting Campers
Explosives are your friend. Claymores/IED's and C4 are probably the most reliable means of dealing with that noob hiding with an LMG around the corner. However, the best way is always to humiliate them in the kill cam (even though they will continue to camp regardless, it's still satisfying).
Killstreaks
Set up your killstreaks to compliment each other. For example, set up an IMS somewhere hidden behind your Sentry Gun to protect it. Also, know your limits on how many kills you can get consistently without dying. It's better to get a Satcom every time before you die than get that Helo Pilot once after an hour of trying.
Get Creative
Prone to see under cars, shoot cars to make them explode, when the sun is just right watch for your enemy's shadow, peek through those extremely small spaces between objects such as ridiculously small windows or damaged walls. Watch your mini map if you see an enemy shoot without a silenced weapon, aim your arrow representing your character on the mini map in the general line of your target and shoot him through the wall. Listen to enemy footsteps on catwalks and the floor above you and you can occasionally get a kill shooting them through the floor. Use C4 to distract your enemies or throw it over a wall and immediately detonate it (this is especially useful when dealing with campers). Hide in those ridiculously odd places in the map (especially in bars, there is always a good place to hide behind a counter in restaurants and bars) when you use a kill streak that requires you to control it. Design your class to always have some kind of trick up your sleeve that other players won't expect.
That's pretty much all there is to it. Just practice, don't expect to be good at something you haven't invested time and hard work in.