Scorestreaks are earned by attaining points during online multiplayer matches. These points are earned completing objectives such as capturing points or killing enemies. The progression of these do not carry over from match to match. You can select which scorestreaks you want to use from the multiplayer menu.
The following scorestreaks count as "Lethal"
Combat Bow (500 Score): A bow with explosive fire arrows
RC-XD (800 Score): Remote controlled car that can be detonated
Care Package (1800 Score): *This drops a random other scorestreak. The scorestreak it drops may or may not be one of the lethal scorestreaks. If you have the Engineer perk on your class, you are able to reroll if you get a scorestreak you do not like from the Care Package*
Sentry Turret (2000 Score): You are able to put down a turret that attacks enemies and their equipment
Napalm Strike (2100 Score): You can target an area for planes to come and drop napalm gas
Artillery (2500 Score): You are given three airstrikes and can choose where they go
Cruise Missile (2600 Score): You control a missile in the sky to crash into enemies
War Machine (3500 Score): Very good Grenade Launcher
Attack Helicopter (4000 Score): A helicopter with machine guns comes in to support your team
Chopper Gunner (6000 Score): You take control of the guns in a helicopter to kill enemies from above
VTOL Escort (8500 Score): You take control of the weapons on a VTOL Jet to kill enemies below
Gunship (10,000 Score): You take control of a gunship and have three types of weapons you can use.
Now use these to kill 50 players overtime (not in one game). This will take a while depending on your skill level. If you are very good, try to use high score scorestreaks as they are better and will add to your total faster. If you are not as skilled at online play, use things like Combat Bow, Sentry Turret, and Care Package. The Care Package is your best bet at obtaining high scorestreaks in you are not as good of a player.