The first step of preparing for a triathlon is to begin to understand the actual requirements of the race. How long is the swim? How long is the run? And how long is the bike ride? Once you've gotten an understanding as to how long it will take to complete the race, you can then begin your training. For each aspect of the race, you should start by walking the distance of the run to get a clearer picture in your minds eye of the reality of running that distance. And in the same way, you'll see what it will be like to swim that distance, and ride that distance. The next step is to actually start training. Depending on your level of fitness we'll assume for the purposes of this article that your body is a good shape already.
With respect to the run, one common misconception is that you actually have to run it. This isn't the case, you can in fact walk if need be. But your training should be along the lines of pushing yourself passed the actual distance of the race run and in that way, you'll prepare your body accordingly to push yourself harder and longer. This will give you the ability to push past the pain as you've prepared for a shorter race. This should also be done with the swimming as well as the riding components. Running is easy, you can run around the block the same amount of times needing for the distance of the race. Some people find it hard to find a swimming location. First start off at your local pool and race laps under supervision. In this way you'll proper your swimming technique as well as improve your ability to swim long distances. Post that, get some experience in the real world scenario of swimming that race. Attempt to get an understanding of what it will feel like to swim in those waters you've eventually race in. Bike riding is easy to deal with. An exercise bike can do the job. An exercise bike also gives you the added advantage of using resistance to build up your stamina more than required for the actual race. It's probably a good idea to mix and match the training between the exercise bike and riding an actual bike. Towards the end of the ironman training regime it can be a good idea to simulate the race by using the same track in the same way that you would as outlined with swimming. Running can also be treated as such.
Also learn to prepare mentally whilst you undergo the ironman triathlon training. Mental preparation is nearly as important as the physical side of things.