I asked a fellow triathlete the following question before my first triathlon: "What's the best kind of road bike to invest in?"
His answer: "Whatever bike you have now is fine."
There's a lot of truth to that answer. The sticker price for a road bike (even an entry level one) can be shocking. For a good road bike that will last you years, one that you can build your skills with, you're looking at something around $1,000 USD.
That's an unnecessary expense, especially if you plan to race sprint length distances.
My first triathlon bike was a Wal-Mart mountain bike that weighed around 4,000 pounds and cost less than $100. I saw someone else at the same race with a pink cruiser complete with a bell and a basket.
As you get into the longer triathlon races, however (e.g. Olympic, Half Ironman, or Full Ironman), you'll want to consider upgrading to either a road, endurance, or triathlon bike.