A good way to improve your skills at MW3 is the Call of Duty Elite website improve section.
It'll give you tips and let you study maps guns etc...
If you haven't signed up for an account you can for free!
Improve Section: https://elite.callofduty.com/improve
If you have any more questions feel free to ask!
to begin with learn the maps, then learn some of the spawn points, then learn the traffic zones. Pick a gun that you like, stay in the outside of the map that way your back is covered. When you comfortable you can try to move to the inside of the map.take cover when you can. this is the basic you will do better, the main thing is not to rage if things aren't going your way. I started with a kd of .64 and now it's 1.57 so yeah practice makes it perfect.
i put up some how to stuff, but tmartn does a lot more (if you can stand him)
here's two javelin videos that'll help you