Now you have a machine gun, don't go into the battle, unprepared.... You've just received your copy of modern warfare 3. Here's the sitrep on where to get started with perks and weapons when you take the fight online.

SNIPERS
Let's begin with sniping. You're probably going to sit and wait for prey rather than go wandering the map.
Recommended Abilities
1. sleight of hand speeds up your reload time, useful for weapons which take several seconds to pop new magazine into the gun.Unlocked at level 4 you won't have to grind hours to try it out. Once you level up to the pro version, you get faster weapon switching, handy for whipping out a second weapon when enemies surprise you at close range and you may need a quick-draw.

2.When it comes to tier 3 I suggest marksmen which is all about range target acquisition. Once you unlock it, the pro version increases the amount of time you can hold your breath to steady your shot.

SUBMACHINE GUNS AND ASSAULT RIFLES

If you're all about running gun, submachine guns and assault rifles are where it's at.
The m16a4 is good to go. With three round bursts, it's large 30 bullet magazine means you won't be reloading constantly and since it's not automatic, it will not keep your crosshair out of the sky and train your accuracy at the same time

If you do want full auto then try out the m4a1. For something smaller the mp5 or ump45, both are good options.
Though you may want to factor recoil into your firing both are best shot in short purse.
Recommended Abilities
1.Regardless of whether you go SMG or assault rifle, both benefit from sleight of hands, reload speed boosts and extreme conditioning helps you to sprint from kill to kill.
2.Quick-draw works well with high firing speeds of these weapons and gives us a slight edge as we bring up your own sites and scope faster.
3.If you find yourself catching grenades, swap to blast shield to reduce the amount of explosive damage you take on the battlefield.
4. Stalker improves your movement speed while aiming down sights and is great for strafing out of enemy crosshairs while firing. The pro version of stalker works a little bit like blast shield but rather than deflecting damage, it delays the boom of planted explosives.

SHOTGUNS
like claymores, like a one-shot kill appeal of sniper rifles, but want to get up close and personal, then shotguns might be for you these boomsticks. It comes in several different flavors and while they can often be slow to fire and reload make up for the sedate pace with explosive fiery death.
The USAS s12 has a slow loading magazine that gets better with sleight of hand while the KSG 12 plays well alongside extreme conditioning but with swift executions.
Recommended Abilities
1.Quick-draw and blast shield are good starting points if you're rocking a support strike package.
2.But if you're more of a lone wolf then using the assault package hardline at level 15 gives you a chance to unlock individual focus streaks one kill sooner.
3.Stalker is still my pick for your go-to tier 3 perk when using shotguns.

LIGHT MACHINE GUNS
Light machine gun share characteristics with their smaller more nimble SMG counterparts, but still much more ammo meaning you don't need to reload them as often. The slow refill times on LMGs makes the sleight of hand essential if you don't want to duck into the bushes whenever you need to restock bullets.
Recommended Abilities
1.Quick-draw speeds up these weapons slow down side aiming and helps to reduce the chances of being shot in the face or needing to go in with inaccurate hip firing.
2.Once again, use stalker to increase your speed while aiming

RIOT SHIELD
If you go with a riot shield loadout, you have to understand that you're probably playing a support role in capturing or holding points and as a result, you're severely limiting your offensive options because you'll need to drop the shields of fire off shots.

Recommended Abilities
1.Sleight of hand allows you to quickly reload and minimize the time out of cover.
2.Blast shield helps keep out straight grenades when protecting objectives.
3.While stalker is good when you first begin but you may consider trading it in for sitrep when you get a few levels under your belt of course.

CONCLUSION
In the end, these are just suggestions, it all comes down to what works best for you and your play style while playing modern warfare 3 multiplayer.