Antman shrinks in comics thanks to the Pym Particles. These particles interact with his suit's technology to alter his size. It's a complex process but crucial to his superhero powers.
Well, a really cool Antman fanfic could be one that imagines Antman in a time - travel scenario. He could go back in time to meet the original inventors of the Pym particles and learn the real origin of his powers. Another interesting concept could be a fanfic where Antman has to fight against a villain who has also mastered size - changing technology but uses it for evil. And 'Antman's Rescue Mission' could be a fanfic where he has to save a group of scientists trapped in a microscopic world.
The first Ant - Man story is mainly about the origin of Ant - Man. It tells how a person becomes Ant - Man, perhaps through some scientific experiments or a special encounter. It might involve his discovery of the technology that allows him to shrink and communicate with ants.
Ant - Man himself is the main character. There might also be some scientists or colleagues who are involved in the development of the technology that makes him Ant - Man.
The key elements are Hank Pym's invention of the Pym Particles. Also, Scott Lang's criminal past and his motivation to change for his daughter. And of course, the Ant - Man suit which is the central technology.
Ant - Man's back story is quite interesting. Scott Lang, the second Ant - Man, was a thief. He stole the Ant - Man suit to save his daughter. Hank Pym, the original Ant - Man, developed the technology. Hank was a brilliant scientist who had been part of S.H.I.E.L.D. but became disillusioned with it. He kept the Ant - Man technology a secret for a long time. Scott, after getting the suit, learned to control the ants and use the shrinking and growing abilities for good, eventually becoming a hero and joining the Avengers.