Call of Duty Guide and Tips
Activision has published a super helpful guide to CoD Modern Warfare, breaking it in three sections:
COD 101: DEFINITIONS OF COMMON CALL OF DUTY®: MODERN WARFARE® TERMS
GETTING STARTED IN MODERN WARFARE®: THE BASICS
THE BASICS OF CALL OF DUTY®: MODERN WARFARE®: LOADOUTS
It is definitely worth ready it even if you are already playing as you never know what you don't know, right?