My advice would be to stay away from attempting the "1337 SNIPEZ" or "QUIK4SKOPEZ20" bullshit.
Find a gun you're comfortable with, learn the maps, find a play style that suits you, and go from there. Take in consideration that some of the players on MW3 have a years worth of experience, so don't get discouraged if you're getting pooped on for awhile. I'd give you a more in-depth explanation, but I'd don't have time as of now.
Cheers.