The triathlon is really two races. The race against fellow competitors and the race to maintain enough fluid and energy in your body to perform at your best. Have water handy in the transition area, carry water on the cycling leg and don't pass up any water stations during the run. When you add up all the accessories that go with triathlon training and racing, it can be a long and expensive list. In the short term, any bike will get you through a triathlon.
Many first timers use mountain bikes or touring bikes.
However if you are thinking long term, buy a bike that fits you - comfort is everything after 20kms or more. See a bike specialist to be measured up and fitted onto the right bike. The more you train, and the longer the races that you compete in, the more carbohydrates you will require. As the transition area of a triathlon is very hectic, be well set up and allow enough time before your race to warm up and stretch. An easy 5 minute jog followed by gentle arm and leg stretches can help prevent injuries.