This is based on my experience. Do you own due diligence:

Things to keep in mind (in no particular order):

Bricks(bike and run) are important
Spend lots of time on the bike
Do allot of low intensity volume
Do as much volume as your diary permit, but grow the volume gradually
never increase volume more that 1.17
Learn proper swim technique. I used total immersion swimming. Many of those who swim fast hate this approach. This worked for me. I was scared of the water and this technique worked well for me
Schedule an olympic distance ahead of the race so you can learn what works and what doesn't for you and to get over your fear of the open water
dont focus on time during your first ironman. Just get through it and get a base-line for future ones
join a club if you can
get a bike fit. makes a bike difference
lose weight if you can. being light matters in triathlon
find a Yoda someone to guide you
if you can do the each disciplines individually, you can do finish the ironman

Things not to worry about:

all the miscellaneous stuff that benefits the elite triathlete: Compression, obsession with gear, obsession with nutrition. These are things that will come later
the swim. its probably what i spent the most time on and what was the easiest part of the race. the wet suit will keep you safe
other athletes during race day. race your race
power meters, turbo trainers, polar, garmin, aero bars, hoka, vibrams. that stuff comes later