Much like everyone else has said, the best way to go is to buy the Battleforce sets. There are also other sets that are more expensive ($150-$250), but they come with a lot more stuff that what you pay for as well. Some of them are called Megaforces or Strikeforces.
That said, you can find a Chaos Marine Strikeforce set from the GW site, or you can ask BG to special order it for you. The Strike force will have over 1000 points worth of models for you to play with, and you will definitely get the most for your dollar.
However, I recommend picking up the Codex and rule book first. Read them over at least a couple of times, before you get into the modeling and painting. Come by the store and ask around if someone can show you how the Chaos marines work too. Get a feel for the game before you invest heavily into it. It's good that you have the enthusiasm to get started, but do take it slowly, as Rich (Trooper) said. It's an expensive game, and you want to be sure you'll enjoy it before putting more money into it.
Good luck!