Iron Man debuted in Tales of Suspense #39, March 1963. Robots had appeared in many comics previously, and even the concept of a man inside a robot suit was not unique. Robotman, a DC hero from the 1940s and 1950s pioneered the concept of a robot suit saving the life of a man who would otherwise have died. So there is nothing particularly original about Iron Man.
Tony Stark, millionaire playboy and ace weapons designer, is in Vietnam testing some devices when he trips a wire and is nearly killed in a grenade blast. A piece of shrapnel near his heart is too dangerous to remove and so he's doomed long-term. The communists, who have captured him, put him to work building a weapon for them. They don't know that he's secretly trying to build a suit of armor for himself that he hopes to use to save his life.
He's assisted in this by Professor Yinsen, a South Vietnamese physicist who opposes the communists. Together they finish the suit just as Tony is about to die. Professor Yinsen sacrifices his life so that Tony can regain some of his strength. Once he does, it's clobbering time! We get some Stan Lee science here:
Art by Don Heck.
I suspect that Iron Man was introduced so that Marvel could say their new superhero team, the Avengers, which would debut in September 1963, was made up of existing heroes; the hope was that having Iron Man, Thor and Antman in one book would also help spur the sales of their main books.
Tales of Suspense #40 continued the trend of Marvel heroes battling aliens in their second outing. One of my commenters (Thelonius Nick?) suggested that perhaps these stories were breathers to allow Lee to decide further what to do with his characters. The story is significant only in that Iron Man decides to paint his robot costume yellow, so it won't be as frightening.
Oh, yeah, that wouldn't frighten a child!