To help Virus out a little with Slaanesh - it's (can't really say he or she) the god of sex, drugs, and rock and roll. I don't think you need to elaborate much more than that. Apart from mentioning that Slaanesh is the god responsible for Lash of Submission, a subject of much anguish for many 40k players these days.
CSM are a great army for starting. They offer a great deal of modeling fun and customization. They also bring a great variety of strong models/units to choose from. In a way that can almost be a drawback to the army. There might be too many decisions to make you might say. If you're going for competitive play, you probably won't find yourself devoted to a single god themed army. In that respect CSM have lost a great deal of fluff. Apart from personal satisfaction and fluff-fun, there's not a lot of reason left to play single god army, in my humble opinion anyway. A Slaanesh demon prince can lead your army, and you can be safe in taking some Nurgle plague marines without any ill-effect that is to say.
My first bit of advice, when starting any army, is spend some time hanging around your Friendly Local Gaming Store. Watch a few games. Your average gamer is friendly enough, I'm sure they'll be more than willing to talk/brag about their army. You've decided to go with an army that's fairly common, so you won't find it difficult to come across a CSM game or two. You may not be able to comprehend all the rules, but pay attention to the interactions of models. Hell, pay attention to which models look sweet. Strike a balance within yourself with how competitive you want to be, how fluffy you want to be, and how awesome you want your army to look. Next decide what style of army you wish to play within the constraints of CSM. Do you want to be more assault heavy? (CSM are already by default a little more assault oriented than their more vanilla SM cousins). Do you want to be a lotta of beef on the board? Do you want gigantic walkers that look like Doom II bosses? (Man, Defilers are sweet) Try to narrow your army down to a definitive core of sorts. Those few models you're absolutely set on. From there proxy the rest. Use blank bases with scraps of paper denoting what they are.
If you're fortunate like I was when I was starting out someone may help you learn the game more hands on before you even buy a single model. My first time was a team game. A friend let me play a couple units in his army against his friend and brother at their dining room table. It wasn't much but it taught me a few things.
Obviously picking up your army's codex at some point is important (if you haven't already), if you have the extra money to drop a BRB might not be a bad investment either. It's important to learn the rules but at its cost you might be better off just borrowing one from a friend or paging through one at your FLGS while you watch a game or two. If your's is anything like the one in my area they have a community BRB, and about a dozen SRB's laying around for communal use. (Some shops frown on spending too much time reading through books on the shelf without actually buying them).
All that said. I'm a strong advocate for a Daemon Prince HQ, Plague Marines, and Obliterators. Chaos Lords are solid HQs, but I personally favor a Daemon Prince. Plague Marines are basically one of, if not the toughest objective camping troop choices in the game. And Obliterators have a silver bullet for just about any unit/model you could want to shoot at. Khorne berserkers are a solid assault troop and you'd be hard pressed to find a list that doesn't run at least ten of them in a Rhino. I also dig Noise Marines, but don't view them as integral as all of the aforementioned units. Of course those are all decisions you'll be able to make yourself after reading up a bit and hanging around the gaming table.
My final bit(s) of advice: Hang around these forums a bit. Never hesitate to ask questions. There's a lot of cool cats, with a lot of advice to give. While you're hanging around check out some army builds some other players run in the 40k Army Lists sub forum.
Lastly, welcome to the forums.
Si vis pacem, para bellum...