Okay so I've never played a CoD game in my life. Which one should I buy to play multiplayer on? I'm thinking CoD4.

Yes, definitely CoD4.

• CoD WaW- Slightly intriguing single player, Multiplayer I do not care for.
  
  
• CoD4- Awesome campaign, Multiplayer is awesome too, Mods like - freeze tag, paint ball, zombies, aliens, promod.
  
  
• CoD2- I've not played CoD2 but I've heard its okay from one of my friends.
  

Edit: Never played a CoD game? :O You sir have not lived.

Modern Warfare 2 comes out in a couple weeks, you should get that one

or cod4 if you insist on getting a used copy or something

Yeah, it comes out on the 10th, but no dedicated servers for PC :(

http://www.gamesindustry.biz/articles/no-dedicated-servers-for-pc-versions-of-modern-warfare-2

I mean, that IWNet thing sounds okay but naaaaaaaaaaaa. Alright then, CoD4 it is.

you can play pirated version of cod4 online just fine, you shouldn't buy their games anyways, or anything from activision for that matter. **** them in the ass. especially don't buy mw2.

Pirate MW2 twice just for the hell of it.

CoD4 will give you the understanding of all of CoD games, and you'll enjoy it the most. CoD2 is easily the best, CoD3 is also brilliant, as is 4. WaW was a massive let down with all the bugs, the annoying over-powered gun of utter devastation, the MP40, and the flying dogs.

I would honestly suggest you get CoD4, then MW2. Even without the dedicated servers, you'll still want to experience the Campaign, if you've played the CoD4 one.
Also, if you do get CoD4, be sure to try Mile High Club on Veteran.