Some Main Points

The little bar statistics for each weapon don't tell you the full story.  Take the M4 Carbine for example, it might not have stats as good as the some of starting weapons, but its fully automatic fire is a very important factor.  The wasted milliseconds between the M16's burst fire can make all the difference. Also an example is the R700 sniper rifle.  It might look like a no-brainer over the other snipers, but consider that it's a bolt-action weapon with a 4-round clip.

You can shoot through solid materials in Call of Duty 4, especially with the Deep Impact perk. Don't forget to shoot through walls.

Once you get the hang of the game, ammo for your primary weapon will be a factor. Keep in mind you can swap out your pistol to pick up a killed enemy's weapon. If you get in the habit of doing this early on, you might never need the Bandoleer or Overkill perks.

Some other Things

Listen Carefully because even if somebody has the dead silence perk on, you can still hear them moving. You can also hear the gun shooting even if they have a silencer on it

Use the map to locate your enemies.

Plant claymores around doors and place one a small distance behind the other, let your opponent see the first one and they won't be expecting the second one.

Be aware of your surroundings all the time – the amount of knife slashing is unbelievable, and quite reliable.

Use your environment, blend in where possible. Remember that opponents will easily pick up on fast movements in the open.

I hope this helps and have fun playing!!