Don't play MW2 is a good solution, but if you want to persist, then I'll give some advices.
Depending on gamemode, you can rack up kills very easily (Although I still recommend going for objectives instead of going out of your way for kills), on maps such as Karachi on Domination, there is a section on "B flag" where you can set up a perimeter where the enemy will usually funnel in one entrance (provided you have either A or C flags), if you check your minimap often, you will know where the enemy is coming from and will be able to respond, getting quite a bit of kills. People will call this camping, but it's more like locking down an area because people are too stupid to find alternate routes. In terms of actual combat, you need to judge on when's a good time to use ADS or hip fire, also when to shoot or when to just say "screw it" and knife them. Know when to rush and when to play defensively and check your minimap.
Depending on gamemode, you can rack up kills very easily (Although I still recommend going for objectives instead of going out of your way for kills), on maps such as Karachi on Domination, there is a section on "B flag" where you can set up a perimeter where the enemy will usually funnel in one entrance (provided you have either A or C flags), if you check your minimap often, you will know where the enemy is coming from and will be able to respond, getting quite a bit of kills. People will call this camping, but it's more like locking down an area because people are too stupid to find alternate routes. In terms of actual combat, you need to judge on when's a good time to use ADS or hip fire, also when to shoot or when to just say "screw it" and knife them. Know when to rush and when to play defensively and check your minimap.