Success in Call of Duty is more about not dying than it is about killing.
- When attacking an enemy, always assume there is another guy.
- Try to get them to come to you; don't go to them. But don't stay in the same place too long. Get a kill or two in one spot and then move on.
- Never run straight up the middle of the map; stay on the outskirts.
- If you're having trouble getting streaks, skip Hardline and take Ghost instead. Like I said above: stay alive.
- Use a suppressor until you understand map flow enough to be able to take the risk of showing up on their minimap.
- If you're using Ghost, avoid your teammates when the enemy has a Spy Plane up. If you're not, stick with your teammates. Never run out by yourself if they have a Blackbird up.
- Use Hacker as your third perk to avoid those obnoxious claymore deaths. You can also use it to find campers (just always throw a flashbang in before rushing the camper's spot).
- Remember that the person who engages first wins 99% of the battles in this game; don't let yourself get caught off guard.
- Shrug off your Halo training and don't go for headshots.
- Never sprint around corners. In fact, try playing an entire match ADSing around corners just to see the difference.
- The two best newbie killstreak setups are Spy Plane/Counter Spy Plane/Napalm Strike, or Spy Plane/SAM Turret/Care Package. If you're using packages, wait until you respawn to call them in (usually the place where you respawn is pretty safe). If you're using the Napalm, wait until a Spy Plane goes up to call it in. Even if you aren't putting up great scores, your team will love you using Spy/Counter Spy planes and SAM turrets.
- Don't use the RC-XD. It's a crutch for players who can't get legit kills; your time is better spent learning how to play the game better.
- Always use tactical grenades to take out Second Chance assholes from around corners; it's not worth risking them getting some lucky or laggy pistol shots in on you. The flashbang, concussion, Nova Gas, and Willy Pete will all kill them if they land near them.
Try using a FAMAS + Suppressor, Ghost, Sleight of Hand, and Hacker. For a mobile class, try the AK74u + Grip + Extended Mags, Lightweight, Warlord, and Hacker. Use claymores (the occasional free kill can be amazing). Work on getting Sleight of Hand, Warlord, and Ghost to their pro versions.
Regarding secondaries; I like the Python + Speed Reloader as the most reliable pistol, with the CZ + Extended Mags as a close second. Avoid the CZ Full Auto (the drop in damage isn't worth the automatic fire). The RPG is a pretty reliable secondary for objective modes as well.
Wow; that was a lot. I didn't intend to type all that out; hopefully some of it sticks and is helpful to you!