Instructions
Play through the main game. Doing this will help you to unlock many of the game's items and secrets.
Play online with other fans of the game. This method will help you to dramatically increase your skills, ranking and levels, and you will be able to unlock everything the game has to offer.
Kill often and kill strategically. Having larger killing streaks will unlock better items and opportunities for you. However, if you run around in the open trying to kill anyone you can find, you will leave yourself unproctected. You have to start your streak over every time you die. Sometimes dying too much can unlock benefits to help you get back on your feet.
Increase your level, or rank. As you gain experience points, you will level up and unlock more weapons. At level 70 you have the option to increase one "Prestige" level and restart at level 1. Unlock all 10 "Prestige" levels for the most elite status in the game.
Complete different quests and get as many stars as possible in Special Ops missions. This will unlock various "Pro" classes as well as trophies for your collection.