Most triathletes have been riding a bike since a child, from the BMX to their first road bike. It seems simple; you get on and pedal. However, as we have learned, bike fitting and positioning is a science. If not done correctly, you may not be maximizing your power and you could harm yourself on the run.
This bike fit example is to show you what goes into a proper one. This particular fit is from a few years ago and has since evolved as positioning and comfort have changed but you can see how many different measurements go into it. You should remember they can get a lot more technical than this with wind tunnel testing and electronic gadgets to determine your optimal position for maximum output.
The key concept to take away is to NOT get on a bike and start riding without discussing proper fit with a professional.