First Ironman is scary. It is a big achievement in your life just to finish one. The preparation is demanding and will take a lot of your time if you want to do it properly meaning to not just finish. Training Peaks offer some good advice in their article.
My biggest advice would be to get a coach, see a sport nutrition, go for comfort over speed in your choice of gear and get a proper bike fit. Mentally, well you just gotta relax and trust the process. No point over analysis, stress and doubting yourself. Just relax and take your fist Ironman as a learning experience. Big titles can be scary so this can help a lot to think about it as just a local race.
We have just proven with IMWA yesterday that you can do very well at your fist Ironman with 100% finishing rate and three 1st timers garbbing a Kona spot for 2020. Have fun and enjoy the experience 🙂