I would strongly recommend that you do a half-distance event before attempting an iron-distance. Preferably do a very hard half, such as the Wildflower, or the Auburn triathlon. It will give you a better idea.
If you are new to endurance and your goal is just to finish within the time-limit, then you should focus on cycling. Especially if you are not a cyclist to begin with. Cycling is the longest event and it is one that offers no alternative. You need to finish on the bike. In the marathon you can walk, but on the bike you need to ride.
Many people worry about the swim, but that should be the least of your worries. If a mass start makes you uncomfortable, you can either choose a smaller, non-WTC event, or you can start way of to the side and far in the back. Smaller events have a lot going for them, and many are every bit as nice as the "real" ironman races.
Either way you will have clear water. Don't worry if you are an extremely slow swimmer, the time alloted is very generous and hardly anyone is disqualified for not making the cutoff. The swim may be a bit tiring, but it isn't going to wreck your day. Chances are it will be over in an hour and a half.
It is important to remember that, even if you take up all your swim time, you still have plenty of time to finish. You only need a 14 mph average on the bike, and you can "run" a 6 hour marathon (slightly faster than walking pace at around 4.4 mph !).
Biking is easily the most tiring event, and although a 14 mph average is sufficient that is faster than untrained commuter speed. So you do need to train. And you need to ride long. I would recommend riding several centuries beforehand. When you do, do one of the following: either try to ride continuously with very brief stops (just fill the bottle and go), and no drafting; Or, ride several 5-7 mi stretches at high speed (22-24 mph). It is Ok to draft and ride a pace-line when you do that. The intensity is what you need. You need to get used to riding long and putting in some intensity as well.
You should be able to ride a century at a reasonable average (19 or higher) and minimal breaks (no more than 30 minutes total), and still feel good afterwards. The best way to judge that is to put on your running shoes and run a few miles. There is no need for long runs, just a mile or two at a good clip will do. If you can do that, you can be confident that you will finish the ironman well within the time-limit allowed.
Just remember, it this is the extent of your training, you will finish well, as long as you don't attempt to race hard. The harder you go, the more likely you are to not finish, or not finish well. That may seem counter-intuitive but it is true. Racing ironman (i.e. going hard the whole time) is very different from finishing ironman (even if you finish with a good time).
Best is to try to finish well the first time. Next time, you will know better and you can prepare for a good hard race. Don't worry, once you do one, you will want to do it again, no matter how painful it was!