Call of Duty 4: Modern Warfare/F.N.G

F.N.G is the first mission you will do in Single Player, it is mainly a Tutorial Level and it suggests what difficulty you play on towards the end.

Objectives

 * Pick Up a Rifle from the Table
 * Enter Station Number 1 and Aim Down Your Sights
 * Shoot Each Target while Firing from the Hip
 * Shoot a Target through the Wood
 * Shoot Each Target as Quickly as Possible
 * Get a Pistol from the Same Place You Got the Rifle
 * Switch to Your Rifle and then Back to Your Pistol
 * Melee the Watermelon with Your Knife
 * Use Your Objective Indicator to Locate Captain Price
 * Complete the QCB Test
 * Debrief with Captain Price

Complete the QCB Test
The QCB test is a test to see how well you will do in the game, it will recommend a difficulty but you are still free to choose any you want. To Start, go to the ladder and look up then go forward, you will climb it, then swap your gun for the   MP5 by pressing the use key, then go to the rope and press the use key, do not jump. You will go down the ladder

Depending on what time you get in the test, the game will give you a recommended difficulty. This table will show you the times.