i don't like the word noob and rarely use the word in game because "noob" is generally applied in a manner that degrades/insults people.
i think babies or newbies are more appropriate terms.
1) ADJUST GAME SETTINGS (especially audio give and receive) — microphones and headsets are always a good idea for they allow you to communicate with team mates and locate direction of zombie attacks. i can't count the number of times newcomers have gotten incapacitated by a lone zombie slashing away at their backs on expert difficulty OR venture off alone and lose their way because they "can't hear anyone giving advice through their microphones".
2) GET FAMILIAR WITH BASIC CONTROLS — E for picking up items, how to strafe L and R, how to heal yourself with a first aid kit, how to pass pills/adrenaline to team mates, how to pick up throwable items (molotovs, pipe bombs, bile jars) and methods of using them.
Note: i recommend practising 1 & 2 on Single Player Mode OFFLINE. that way, you get to learn the basics at your own pace without incurring the wrath of unpleasant players online.
3) PLAY CAMPAIGN, pit your skills against the AI Director to start with — there are several game modes to choose from and as a general rule of thumb, you can either start with SINGLE PLAYER on normal/advanced difficulty and complete basic maps with AI bots... or play CAMPAIGN on easy/normal difficulty with other players ONLINE.
4) PRESS TAB — tab is good. the tab button gives you an overview of useful information about the server you've landed in: your ping, game difficulty, name of the campaign, map progress.
maybe I'll add more to the list later.
meanwhile, these are useful guides that helped me personally when I first began playing l4d2: Proper Item Usage
By RestlessProper Throwables Usage By Restless
Left 4 Dead (Basic Tips on Expert) By Michael Endohttp://www.youtube.com/watch?v=z6tnAUsGufk
Survivors: Basic Strategies and Tactics By topmo3vids http://www.youtube.com/watch?v=Yaw9G-wvgXA