Adrian,
Ohhhhhhhhhhhhhhh I get all giddy when I hear someone who wants to become a tri newbie. To find some good information you can go to www.beginnertriathlete.com or www.trinewbies.com. I believe the tri newbie site has some free training programs for different distances (sprint, olympic, half IM, etc). I used the half IM distance one last year and really enjoyed it.
First, what better place to start than the START!!! Since you are a competitive swimmer, this leg should probably be the easiest for you. However, you do not get your own lane and it is a fish eat fish world out there when the race has a mass start. You will get elbowed, kicked, grabbed, pulled, swam over, under, around and sometimes even through. WHAT FUN!!!! Other races may have wave starts where you go with a group of say the same age group. Either way it can be chaotic so be prepared. If you are fast though, you can get out front and avoid the melee. The BIG difference for you will be, there are no lines on the bottom of the lake or ocean to follow so you need to learn how to sight so you swim in a straight line. That means every third or fourth stroke you need to glimpse forward to spot a buoy and it also means bilateral breathing.
Practice you transition from swim to bike in your driveway, bedroom, or even your head. You can visualize your T1 anywhere so the more you practice the smoother it will go. There was an article written about T1 set up on the Beginner Triathlete site but you may have to look for it. If not, you can probably search on the internet and find an article about it or just ask others how they set up.
The bike is pretty straight forward but remember the rules of the road. Stay to the right and pass on the left. There is no drafting in USAT races so stay back from the rider in front of you the proper bike length until you are ready to pass them. You have I believe 10 or 15 seconds to pass, so ride up on their tail and get a little draft and then whip around them. Once your front tire passes theirs, they have to drop back into the non-drafting distance mentioned earlier. Just be considerate of others and have fun. I personally, like to talk to folks as I am passing them or cheer them on and give them encouragement.
T2 as mentioned before is a different monster. The legs will be a little funky feeling once you get off the bike, put on the shoes, and head out on the run. The BRICK training, again as mentioned before, will definitely help your legs become accustomed to the change. Truck along and enjoy the run and if you can, give more encouragement to those you pass or those that may pass you.
Once you finish your first, you will be HOOKED and will officialy become both a Triathlete and a Tri Geek because you will start looking for items that will make you faster in each leg. (like a new pair of aero wheels or a new bike)
Have fun and keep us informed on the training and your first race. We love to hear race reports!!!
Jim C2