You've registered for an Ironman before you got a bike or even learned to spell the name of the sport!!???
That qualifies as either the height of arrogance or complete lunacy. Please tell me that you mean a sprint distance triathlon and not full Iron distance (2.4 mile swim/112 mile bike/26.2 mile run)!!!
Rant over.
My suggestion would be to start training with whatever you have now, and plan on doing some short course races for a couple years to get a feel for the sport before making the jump to Iron distance. It takes a lot of hours and miles to learn enough about endurance racing vs. your own physiology to make a competent effort at that distance. Hook up with a local club. You can learn a lot by hanging around and training with more experienced racers. Believe me, there's a LOT to learn. See links below for more information on clubs, rules, etc.
Back when I started, I had the good fortune to stumble across a group that included a couple of young pros to teach me the ropes. Even then, I raced shorter distances for a few years and did several marathons before I felt ready for the big one. The payoff was a race with no surprises, a relatively good result, and no doubts that I was going to finish.
-------
Edit: I know I didn't answer the bike question. Follow the suggestions above, and the bike Q will likely answer itself.