Heya 50 cent! I got a few pointers, some of which people have mentioned already.
1. You need to find out what play style suits you as a player first. As Heretic asked, do you like to sit back or do you like to run and gun? This is the first determinant of what guns to use etc. Craft the perks and guns around your play style.
2. Picture and play the game as if you were playing in real life. If you were playing in real life, would you stand out in the middle of the road and shoot at someone behind cover? Of course not, unless you wanted to get shot. Try your best to give yourself the advantage and the enemy the disadvantage. Environmental awareness is a massive advantage that is not leveraged by a lot of players. Use cover, make the enemy take low chance shots while you take high chance shots. Examine your posture, if you are standing up, you expose yourself more as the enemy has more surface area to shoot at. How much of your body is being shown etc?This game is just as much mental as it is physical which leads to my next point.
3. Try and out think your enemies. If you look at the UAV and you see a massive amount of red dots in a certain location, a good strategy would be to try and get around and flank them if you are a run and gun player. If you are a position player put your self in a choke point where you can try and pick people off. Some people try to be in a position where they can see multiple paths. I would not try and do that personally because it takes away from your focus. I usually try and put myself where I can see 1-2 areas clearly so I can focus on those. The more areas you can see, the more areas you are exposed to and can get shot from.
4. The UAV is your friend. Learn to use the UAV effectively. Glimpse at the uav every so often to get a sense of where the enemy is as well as your team. If you look at the uav and see very few green arrows in your area, more than likely the enemy may soon be coming around to you so you will have to be vigilant.
5. Try get some kind of perk edge if you can. I know I spoke about giving yourself an environmental edge and one in terms of posture (if possible) but depending on your play style, there are some perks that can give you a slight advantage. For example, uav jammer is a great perk if you like to run stealth. This gives you the advantage of surprise so that you can rush in and take out a lot of players without them realising. If you are a pocket player, you may like juggernaut and bandolier which give you extra health and ammo. Use perks towards your game style.
6. Practice As Ron and SSG have said! You will need to practice to work on your aim, reflexes etc. Watch other players and analyse what they do and why do they do it. Why did they take cover at a particular place? Why did they throw a grenade at a particular spot? You need to learn how to read how other players play. For example, if I am playing and a particular player likes last stand, if I encounter them, i will make sure to try and change my tactics. Or if they like to lay claymores, remember their spots and try and trigger it before rushing in. Remember also that this will count towards you. Change up your spots and processes from time to time or people will be able to read you easily. There are a few pocket players on our server who can be devastating if allowed to get into position and you do not know what to do. Some people complain about it, I just throw a grenade into their spot. They are easy to kill because they use the same spots every time. Adjust and adapt.
Hope this helps!