First step is to read some Chaos Tacticas. Get an idea of how the army works, what you want it to do and which units are simply not up to scratch. A great example can be found:
http://www.lounge.belloflostsouls.net/showthread.php?25877-Chaos-Space-Marines-Tactica As a buyers list:
You need a
HQ and 2 Troops. Chaos is in an odd spot because your
HQ directly affects which units you can take as Troops, and so we'll start on the Troops:
Cultists: The favourite 'default' troop, these guys exist to either form a Fearless meatwall escort or to hide in craters and claim objectives. 4pts goes a really long way when all you need is a Scoring body. Keep them cheap, keep them simple and just run them without upgrades. You need a Champion model in every unit but other than that, just keep them basic with
CCW and Pistols, don't upgrade to Lasguns and certainly don't get Marks involved. A Flamer is alright if you find yourself with spare points at the end. Normally taken in minimum squads of 10, or go all the way to 35. Very little point in taking a mid-sized squad.
Chaos Marines- The cheapest Marine profiles going, unfortunately you get what you pay for. Generally held to be overcosted relative to Cultists, Cult-unlocks and other Marine Codices. If you want to stick them out I'd roll in 10-man squads with double Plasma Guns and slap a Combi onto the Champion. Marks are not advised unless it is Nurgle, and even then you'd probably be better going the whole hog and just taking Plagues.
Noise Marines- Require a Slaaneshii Lord to take as troops, but really nice as a unit. Excellent firepower that denies Cover, decent assault potential and the option for
FNP all combine to make a very versatile, very efficient package. Can be run either as assault units, or as shooting: Shooting always get the maximum number of Blastmasters with Sonics optional, whereas assault units always get the Doom Siren and
CCW/Pistol. Consider the Icon in larger squads but it's a bit pricey for the min-sized ones.
Plague Marines- The big boys of the Codex and the best defensive infantry in the game. Extremely simple application, just take the maximum allocation of Plasma Guns and find a nice objective to squat on. Equally proficient in shooting or assault and ludicrously hard to remove, they excel at making themselves very bad targets and so go a long way towards winning objective games. Lack offensive power and so typically run with aggressive support units to actually get the killing done.
Regarding your
HQ, I'd keep it simple for now and work with the Lord or Sorcerer.
Lord- Cheap beatstick, confers Fearless and required to unlock Noise and Plague Marines. Extremely versatile options with the only mandatory upgrades being a Mark if you want the relevant Cult troops. Taking a Bike or Steed is highly recommended, as is some form of melee weapon. The Burning Brand can be an excellent tool and there is very little reason not to take a Mark, even if you don't want the relevant troops. I'd always try to find an escort for him, either being Cultists when on foot or using Spawn/Bikes when mounted.
Sorcerer- Always Mastery 3 and always Unmarked. What you lose in slapping things, you more than recover in terms of psychic support. Telepathy is your best bet here and the best reason to avoid Marks, because the Chaos disciplines are very mediocre. Again you need a good reason not to slap him on a Bike, but you can get away without upgrading his melee weapon. I'd still get him an escort and would prefer Bikes to Spawn in his case.