Here's my simplest advice.
First, get yourself to a bike shop. Look at the models, check the prices, and have a budget in mind. To be honest, a new road bike is expensive. I'm talking, out the door for $1500 is a great deal. Partly because along with the bike you'll need shoes, pedals, a bike helmet, padded shorts, a saddle bag and extra goodies like that. It's like adding a la carte sides to your steak from Ruth's Chris - you've got to have your garlic mashed potatoes, that's what makes the meal.
Back to the task. Go talk to the pros at your local shop. Tell them what you are looking for. Digest all that advice they are going to give you and you'll begin to narrow down your choices and things will become clearer for you. Ask about what size you might be. See what they have that would work for you. Tell them your budget!
If you can afford it, a new bike perfectly fit for your body is a sweet purchase that will get you on your way to your future triathlon career. It is a great investment. But if it is beyond your budget, tell the shop pro. Ask him about re-sales, trade-ins, or other options, even financing. Go to him for help!
Another route is to get the word out to other cyclists that you are in the market for a new bike and see what's out there. If you find what you are looking for on Craig's List or Ebay, MAKE SURE you bring the bike in for a tune up from a reputable bike mechanic. Ask him about getting properly fit on your new ride. A bike should be comfortable, with a seat that does not chafe your fanny. Your arms should not go numb when you are on the bike for a long distance. The ride should be smooth and quiet and blissful.
The most important part of this whole process is finding something you will love and something you will want to ride for years to come. My first road bike was a Specialized Allez. It was a mid-range bike that got me through my first years in the sport. I put aero bars on the bike when I did my first tri and guess what, I loved the bike AND the race. And now that bike has been passed down to others for their first races.