1 - PRACTICE Play Boot Camp. This will give you the chance to play all the different gamemodes to work out how to play them properly without ruining the game for anyone else. If you're the guy going 4-33 in a pub match you're always going to be that guy until you hone your skills in an easier setting. You only get half XP but that shouldn't matter since you'll be getting more XP than normal it's also great for levelling your weapons and trying new things.
2 - CHOOSE YOUR WEAPONS Try to give all the guns a go, try to get 50-100 kills with each one to see which ones you like best, you've got 5 classes so use them. Choose the attachments that best suit you not the ones you heard/read were the best, not good with the irons? Try the sights out, get caught on the reload a lot? try fast/extended mags. Remember that some weapon classes are better suited to different maps.
3 - PERK SET-UP Use perks that are relevant to the game mode and the way you are going to play it, it's no use repeatedly jumping on Dom flags with Ghost+Hardline and Ex Con when you should be using Flak Jacket and possibly Tac Mask. I would suggest you run Toughness on every class I can't stress how important a perk it is.
4 - SCORESTREAKS Use obtainable scorestreaks. I would recommend you DO NOT run the Care Package, not only are you taking yourself out of the game to wait for it but you're a sitting duch while doing so. UAV-CUAV-Lightning Strike is a much better low set up and is useful immediately.
5 - MAP AWARENESSDon't constantly run through the center of the map, work your way around the sides of the map and cut through to the center if you need to get there. Try not to sprint too much and if you're about to round a corner take advantage of the headglitch Treyarch placed there and use it as cover while you ADS round it. Always pay attention to the mini map but don't just chase the dots use it to get an idea of where the enemy is and plan your approach accordingly.
"U MAD?", "Get good" & "Use x perk" are legitimate responses to everything.