Start slow...it is easy to ramp up fast! Since you already know how to play, I would just get any of the intro packs from INN block (innistrad/dark ascension/avacyn restored), whichever appeals to you, and learn to play it. Then crack a few boosters - you'll get your first with the intro deck - incorporate them, start experimenting to expand the deck you already have, maybe pick up a second intro pack for variety. Ask for any commons your friends can spare, most people have loads lying around gathering dust, and this will greatly expand your customization options without any financial investment. Honestly, you can do quite a lot with one intro deck, a few boosters and a big stack of commons, and I'd just swim in that pool until you feel like you have a good handle on deckbuilding. If your friends are particularly generous (and want you to stick with the game), they might even throw a few spare uncommons or rares your way.
Once you have a pretty good feel for the game, consider participating in a draft or limited event, even if it's something casual with a few friends (or even
one friend, I've been playing a lot of
Winchester lately, great fun). You will lose a lot, but it's a great way to learn the cards in a set, imho. Some newbies don't like draft so much, though, because winning requires a decent grasp of the
color pie. If you do it a couple of times and don't like it, there's no reason to continue, but consider returning to it once you have a cycle or two under your belt (assuming you stick with the game for awhile).
I strongly recommend developing your own decks, but it never hurts to see what's winning, especially if you want to extend your competition pool and start attending FNM somewhere. The official site and
mtg salvation are both great resources for that. Try building strictly from your available card pool for a little while, but eventually you'll want to peruse all the available cards in your format (again, likely standard), or at least in the block you're learning, to determine which cards you really need to make your deck sing. Buying singles is often much more cost effective than cracking boosters, but determining what to buy can be intimidating before you are familiar with a block. It's easy to plunk down money on an expensive card that doesn't actually work as well as you thought it would, so I'd urge you to post your early deck lists here for people to critique and help you prune.
I'd probably stay away from 2012 intro decks since 2013 is right around the corner, and some of those 2012 cards will be leaving standard in a couple months. That might not seem like a big deal right now, but you will probably end up caring about winning within the boundaries of standard formats before too long. I'd avoid the Scars block (Scars of Mirrodin, Mirrodin Besieged, New Phyrexia) for the most part, too...Innistrad block has plenty of great cards and decks for you to experiment with while you wait on Return to Ravnica, when Scars block cards and 2012 cards not reprinted in 2013 will leave the standard format.
I always forget about the event decks...those are great too, but mostly you're just going to want to pick a deck that looks like a lot of fun to play....event or intro, your call!