Jeff,
Rblount has a pretty informative post above. However, I'll give you my 2 cents should they differ. Had about 5 seasons of glory days in the late 90's early 2000's. Was just a middle of the packer but placed a few times now and again. Got up to almost a half iron, basically a high olympic.
1) The leg that tends to separate the "men from the boys" is the swimming. Because anyone can run and bike. But swimming has just so much technique and form. It's not all just wind and strength. Also has a way of putting you severely behind from the get go. You need to swim a) open water and b) see if you can swim w/ friends or at least be prepared for what lies ahead. No one will be able to prepare you for the chaos that ensues in the beginning of the race during the swimming leg. You WILL get kicked, elbowed, pushed under, etc. Just stay focused and try to get in the zone early. If you find yourself intimidated or out of breath, it's probably not your preparation, it's probably the 100 or so people swimming around you . It's really hard to explain. It certainly takes a little while to get used to. The stronger the swimmer you are (and more winded), the better off you will be. But if you have to do some breast strokes or side strokes to catch your breath or get back the lay of the land, do it. Nothing to be ashamed of. If you aren't confident in your abilities, make sure you line up in the back of the heat. Because again, you WILL get pushed straight under. The "mad dogs" will be going after it. Remember, on your first race - that's not you.
2) BRICKS are highly valuable. Like already mentioned, you need to get your body used to running right after cycling, period. It's just not natural. Your legs will feel like cement. The more you practice this, the less like cement they will feel as you will become more adept at removing the lactic acid and what not. And will be more able to bare down and get into your stroke.
3) Get into a training regimine/schedule if you haven't already. Like solid, unmatched planned workouts. They have coaches for this type of stuff too of you feel this is necessary. You have to train smart. If you're not giving yourself enough rest, or training the right techniques, it doesn't matter how much you train.
4) You haven't mentioned your ht/wt. Lose weight unless you're a skinny guy. You don't see too many fat/big boned triathletes. Think of it this way, the more you weigh, the more you carry around w/ you for a 1/2 m swim, 12 mi bike, 3 mi run....... You'll be doing yourself a favor to get fit and lose as much as possible.
5) Be prepared. Plan ahead. Prepare yourself mentally for the swim. Prepare your transition station w/ your sneakers/gear/sunglasses, etc. You don't want to have to look for stuff. Rememeber what ROW you're bike is in. Nothing worse than spending 15 minutes just in the transition area because you forgot to leave one of your shoes in the transition area or forgot where you put your bike among 600 others... Looks at the entry and exit part of the station to get aclimated.
6) Have fun. It really is a blast. Think about it, your body is really the only thing you truly own. Figure out why you're doing this. Is it a test? Bored? Trying to prove you're not old?
LMK if you have more specifics. Again I was just a middle of the packer, but I sure had fun for awhile there.
Oliver