Tip 1: Play with gun that fits you. Its always good to no that your comfortable with any gun. I would recommend to use Sub Machine guns, or Assault Rifles.

Tip 2: Always play the objective, if you like getting kills, go play TeamDeath Match,or Free for All. You got to think about the other people on your team.

Tip 3: Don't worry about your stats, they aren't everything. The game is all about fun, not to see who is the best.

Tip 4: Play on a comfortable sensitivity, it makes a big difference. Best way to check, is to go into a private match, and going through all of them.

Tip 5: Memorize the Maps! This helps a lot, making a route in the beginning of a match can lead to a lot of kills.

Tip 6: Know the spawns. Spawns are very important, each game type has different ones. Knowing them will give you an advantage, and will give you more wins. You can find spawns here: [ Register or Signin to view external links. ]

Tip 7: Beware of your surroundings. Looking in one direction will easily get you killed. A good way is to think that you are in a dark alley, and you are looking every were so you don't get killed.

Tip 8: Play MW3 in a comfortable environment. Sit in a nice chair, grab some snacks, and relax. Did you ever notice that you sweat when you play? That means you are not relaxed, its either your raging, or you have some problems.

Tip 9: Use good perks, and equipment. For equipment use either Frag, or Semtex. For perks use something that is obviously comfortable to you. But don't use unreasonable perks.

Hope this helps.
Note: These tips aren't for everybody. A lot of these tips are obvious but could be helpful to others.