I'm excited to see it because I feel like no one ever asks this question and I believe I have a rather good answer for you:
If you're into comics you might have known to expect that the best YEAR ONE scenario for Ole Shell Head to come from the man who knows more about obscure details, characters and back-stories than any other comic writer: Kurt Busiek
Aside from being a big comic historian Kurt is a long time and well celebrated comicbook professional who has written just about every kind of story. His Astro-City series has won tons of awards and has possibly one of the best Batman & Robin like stories in it I've ever read.
You can find Iron Man: The Iron Age collected in Iron Man By Kurt Busiek and Sean Chen Omnibus or just pick it up a at your local comic shop as it's only a two-issues prestige format mini-series.
And don't just take my word on how good it is Comicbookresources.com called it great as did Comicbookrevolution.net.
I hope you enjoy the read which starts with Tony in his original armor and ends with him in the more traditional red-and-gold. It not only gives you details on Tony's early days in the armor but on the side characters that populate his world and it is told in a way remnant of Kurt's work on Astro-City.