for kill streaks press the rt d-pd button

Killstreaks become available when you have killed enough people to pay for them.

You will get a voice-over telling you the package is ready to use. If you are playing on normal modes (not hardcore) you will see your killstreak stack on the right of the screen.

If you rarely get more than a couple of kills before dying yourself, go for the support package option as you will get rewards based on the total number of kills you have in the game, not just in one life.

If you have multiple rewards available (including ones you've stolen from enemy care packages) you can select which one you want by pressing "up" on the d-pad, then "right" t activate it.