Method to level up:This method is pretty simple and very effective
levels 1-15: levels 1-15 my method is use the support strike package using UAV,SAM Turret and Advvanced UAV and when you can use Counter UAV when you can unlock it as this means you can get all your killstreaks quicker and go through them again and therefore gaining more xp.

levels 15-20: levels 15-20, you now have just unlocked hardline which in my opinion is the best perk on the game and the most useful perk to level up. Basically 15-20 just use hardline to gain your killstreaks quicker and you will get to 20 in about 1-2 games.

levels 20-80: levels 20-80, this is where the most xp is gained, at level 20 you have now just unlocked the strike package, speacialist. the specialist strike package is very effective with hardlinne, simply select the perks you want with your strike package. By doing this in a game you can have 6 perks and if you get a 7 killstreak you now have all the perks in the game. each time you unlock a perk you get 200-300 xp, when you gain all the perks you get a specialist bonus providing you double xp on your kills.Specialist is by far what you need to level up fast.

[Tip]with your prestige point every time you prestige, you should use it on double xp, my advice for this is wait until you are about levels 40-70 to use this because you will have unlocked and made your best classes so therefore you should do better and gain more xp then if you use your point straight away at the beginning of the prestige.

Classes:
by far the best classes to use is SMG's so you can run and gun getting the most kills if you play sensibly with them.

classes is use-

levels 1-28:
primary: UMP45 (kick)
secondary: FMG9(singluar or akimbo) ps. akimbo is beast!!! recommended
Lethal: frag
Tactical: concussion
perk 1: Sleight of hand
Perk 2: Hardline(when reached level 15, 1-15 i used quickdraw)
perk 3: Stalker

levels 28-80:
primary: PP90M1 (kick)
secondary: FMG9 Akimbo ps. akimbo is beast!!! recommended
Lethal: frag
Tactical: concussion
perk 1: Sleight of hand ( also use scavenger to get challenge xp)
Perk 2: Hardline
perk 3: Stalker,Steady aim or dead silence

Game modes:

There are mainly to game types i play

Gametype 1- Team defender:

Great gametype, lots of kills and lots of xp, if you team has the flag you get double xp. I would play this with a minimum of 2 friends so you are with people that they know what they are doing.If you run and gun with SMG, you will easily get 25+ kills a game, averaging about 10,000xp a game.

Gametype 2- Search and destroy:

Great gametyple, i wont explain this because most of you will know this gametype as it is not new in this call of duty. 500xp per kill and 1000xp for a defuse or plant, i suggest rushing in and getting as many kills as you can in as little time as you can because when you are dead your gametime will not increase.only play this gametype if you can average around 7+ kills a game, i prefer playing this gametype on my own for more xp

Thats all hope this helped, this took me a while so please thank the topic and maybe post of what you think of it ( rep appreciated)